Slowly on to the roof of Africa: At 5,000 metres on Kilimanjaro it is minus 15C. To keep warm you have to move quickly; if you move quickly you get acute mountain sickness. David Belton reports

Halfway up the escarpment, Pete, a Californian computer analyst, begins to throw up. His whole body shakes as he brings up streams of the sterilised water we have forced down our throats all night. A few feet away, Rob stands hunched against the cold. His face, eerily lit by my torch, glows white. After four hours' hard climbing, Gillman's Point looms 300m above us. I ask Pete if he thinks he can make it.

'I'm fine,' he gasps. 'Let's just get on with it.'

We trudge up the side of the mountain, our boots crunching on the frozen scree.

'Pole pole,' whispers our guide in between Pete's retches.' Must walk slow. Still got plenty feet to climb.'

According to the Mountain Club of Kenya's guidebook, low oxygen in the air - hypoxia - is 'noticeable above 3,500m and marked above 5,000m'.

The book, which has become an extension of my arm since my arrival in Tanzania, warns that the thin air will result in almost all climbers suffering from acute mountain sickness (AMS). Symptoms include 'headaches, nausea, vomiting, anorexia, fatigue, insomnia, swelling of hands, feet or face, and decreased urine output'.

For most of the amateur walkers who pour into the dusty dishevelled town of Moshi, climbing Kilimanjaro represents the pinnacle of their mountain careers. At 5,895m, the summit, Uhuru Point, has half the oxygen available at sea level. Few who gather in the YMCA dining-room the night before the trip have climbed at such altitude. Understandably, the atmosphere is tense. On average, three-quarters of all climbers are struck down with AMS before they reach the summit. The illness is indiscriminate, affecting the fit and unfit in equal numbers.

The Mountain Club advises three antidotes to AMS: 'Descent, descent, and descent.' This is all very well if you have reached the summit, but dispiriting if you navely believe, as we did, that paying a minimum charge of pounds 215 to the Tanzanian government to climb Africa's highest peak gives you the right to reach the top.

The dining-room is transformed into a kind of international medical seminar in an effort to discover the best ways to avert the desperate action the guidebook suggests. Advice is freely exchanged: drink plenty of water, load up on carbohydrates and fruit and, most important of all, walk 'pole, pole' - slowly, slowly. The theory is simple: a slow walk allows the blood more time to acclimatise to the increase in altitude. The grizzled faces of those who have returned from the summit nod wisely - and just a shade pompously - in agreement.

Only one German couple disagrees: 'Zis altitude sickness, Reinhold Messner says it is just a state of mind.' Pete tells them he would hate to guess at the state of mind of a man who has climbed every peak over 8,000m on his own without oxygen. The Germans roar with laughter.' Ha] Ve vill see]'

They continue eating their chicken and rice stew with a relish that did Tanzanian YMCA cooking more justice than it deserved. I consult my book: 'A diet high in fresh meat and carbohydrates seems to help reduce symptoms of acute mountain sickness . . .'

The 50km Marangu route is the easiest and most popular climb to the summit. The first night is spent at Mandara hut (2,700m) and the second at Horombo hut (3,700m). These alpine-style chalets were built by Norwegians and have solar-powered lighting and flush lavatories. Each night the camps are a hive of activity as guides bark orders at their porters, who scurry between the main chalet and the cookhouse with steaming piles of chicken stew.

From Horombo, walkers cross the huge Saddle - a flattish desert wasteland - to Kibo hut at 4,700m. Then, in the early hours of the morning, the individual parties make the final ascent up the viciously steep scree wall to Gillman's Point, which sits on the rim of a vast crater. With the hard work over, there is a two-hour, 1.5km walk around the crater to Uhuru Point. A piece of cake.

If the walk through the forest belt that wraps itself around the lower part of the mountain is relatively easy, the steady trickle of exhausted climbers coming down the path is a constant reminder of what lies ahead. At Mandara hut, a group of Japanese students stumbles into the camp. Their guide shoulders one who is only partly conscious. His legs appear boneless, like those of a marathon runner who has collapsed heroically just yards from the finish. Thomas, the guide, grins at me: 'Many people fail.'

The 14km to Horombo are even less arduous than the previous day, but by mid-morning Pete and I feel mildly hung-over. There is a nagging throb behind my left ear. Both of us are anxious. If we are suffering at 3,000m, how will we feel at double the altitude? We slacken our pace and drink more sterilised water. I check my book: 'Clear and copious urine is always a good sign that you are drinking enough liquid.' This is a relief: I am sweating too much for it to be copious, but at least it is clear.

Horombo hut also serves as the overnight stop for those who have attempted the climb to the summit in the early hours of that morning. Inside the chalet, Frank sits morosely picking at his stew. Seventeen years ago, on his honeymoon, mountain sickness forced his wife to stop an hour away from Kibo hut. This time, with his wife back in New York, he had made it to the crater rim: 'It was unbelievable. I felt terrific at Gillman's, but half an hour later - bang]'

We stop eating.

'Felt like those Iraqis. Blew the shit out of me.'

At the next table, a group of French walkers are toasting their victory.

'Good for them,' says Frank, 'damn glad some of us made it to the top.'

Good for Frank more like.

It was not until the walk across the Saddle moonscape to Kibo hut that we feel any real physiological change. Hands start to swell, heads pound and the air is noticeably thinner. Our steps become smaller and metronomic. Despite the gentle gradient, the 13km takes six hours.

At Kibo hut, the relaxed chat between walkers during the previous two days disappears. Above lies Kibo, a 1,000m scree wall rising out of the mountain base that we will face later that night.

Everyone feels sick and listless. Across the table a Spanish couple take turns to throw up into a bucket. Tagisha, a chemistry student from Osaka, leaves his orange behind. Our eyes widen as he disappears down the corridor. When he returns to reclaim his property, we stare at our shoelaces in shame: an orange left behind at 4,500m seems to have the same value as a set of keys left in a car door at sea level.

The temperature falls to -15C on the scree wall. The only way to keep warm is to move quickly, but this will bring on altitude sickness and put an end to the climb. We resolve to continue our freezing crawl. I remember the advice in my book: 'Leave at 3am, because it is a good thing not to be able to see the stretch of scree ahead of one.' Looking far above at the flickering lights of other groups still on the scree wall, I feel cruelly betrayed by my mountain bible.

An hour later we pass the Messner fans. Both are in no shape to go on and Thomas confers with their guide to check they need no help to get down. The end of the Messners perks up our computer friend. Pete tells us cheerfully he has read that what we are doing is the equivalent of climbing nine Empire State buildings laid end to end at a 16 degree angle.

Gillman's Point. After all the cold, vomit and blinding headaches, the scree wall is beaten; the summit is close. Patiently, Thomas tells us not to rush it, just push on, pole pole. The sun has yet to appear through the vast bank of cloud that lies 4,000m below us, but Uhuru Point is now clearly visible in the early dawn.

We reach the summit two hours later. It is too cold and we are too exhausted to grumble that an overcast morning has deprived us of one of the best views of Africa. In the log book - where our swollen hands proudly scratch out our names - someone has quoted John Rebman's first words when, as a missionary in 1848, he became the first Westerner to set eyes on this snow-capped mountain so close to the Equator: 'He hath shewed his people the power of his works, that he may give them the heritage of the heathen.' (Verse 6, Psalm 111.)

Getting there: Africa specialist Bestways Travel (071-930 3985) has a return fare from London to Kilimanjaro airport, 30 miles north of Moshi, for pounds 475. It also offers a return with BA from London to Dar es Salaam from pounds 459.

Accommodation: In Moshi, the YMCA has clean, basic rooms and a restaurant. At Marangu, where the trek starts, there are two slightly smarter hotels. The Kibo has 75 rooms, each with a private bath. The Marangu has 30 rooms: no private baths, but the hotel does delicious food.

Guides: It is mandatory to hire a guide to the summit. In Moshi, the YMCA runs the most regular and cheapest climbs. They provide guides, porters, food and any extra clothing. There is no need to book in advance. The trip costs dollars US300 ( pounds 215) and must be paid for in US dollars. Both the Kibo and Marangu hotels run trips up the mountain, but they charge as much as dollars 500. It is polite to tip the guide dollars 10 and each porter dollars 5 at the end of the trip.

Kit: Clothing can be hired, but tends to be quite old. A warm sleeping bag, walking boots, a fleece jacket, woolly hat, sunglasses and warm gloves are essential. Gore-Tex gear smacks of overkill but is vital if it snows. A torch is handy for the summit climb, and it is important to have a couple of water bottles and plenty of sterilising tablets - understandably the guides are unhappy about carrying your water to the summit.

Fitness: Fitness does not guarantee a successful climb, but it is as well to be strong before arriving at Moshi. Being fit increases your air intake and thus your chances of making it to the top. Do not climb if you have a cold, sore throat or a fever.

Time to go: Kilimanjaro can be climbed all year round, but avoid the rainy seasons (mid-March to early June and November to December) when the snowfall is heavy.

(Photograph omitted)