Acute mountain sickness (A.M.S.) and its severe complications, high-altitude pulmonary oedema (H.A.P.O.) and cerebral oedema (C.O.), were studied in 278 unacclimatised hikers at 4243 m altitude at Pheriche in the Himalayas of Nepal. The overall incidence of A.M.S. was 53%, the incidence being increased in the young and in those who flew to 2800 m, climbed fast, and spent fewer nights acclimatising en route. It was unrelated to sex, to previous altitude experience, to the load carried, and to recent respiratory infections. The severity of A.M.S. was inversely related to age (independent of rate of ascent) and the highest altitude attained, and was highly ocrrelated with speed of ascent. There were 7 cases of H.A.P.O. and 5 with the more intractable C.O. and, of these 12, 11 had flown in, 9 had spent only one night at Pheriche, and none were on acetazolamide. 11 required evacuation. Acetazolamide, compared in a double-blind study with a placebo and also compared with no tablets at all, reduced both the incidence and the severity of A.M.S. in those who flew to 2800 m but not in those who hiked up to that altitude. Prevention consists in slow ascent, rapid recognition of warning signs, and prompt descent to avoid progression.