Twenty-one climbers who were members of the American Ultima Thule Everest Expedition participated in a double-blind, randomized clinical trial of phenytoin prophylaxis for acute mountain sickness during the approach to the northeast ridge of Mount Everest. The study was carried out between Beijing and base camp at 16,800 feet. Time spent ascending from Beijing to base camp averaged 13 days. High-altitude symptom questionnaires were filled out beginning in Lhasa at 11,800 feet and in Xigatse at 12,000 feet, in Xegar at 14,000 feet, and at base camp. Computer analysis of the questionnaire answers performed by an impartial analyst revealed that climbers who took phenytoin were less likely to have headaches at base camp. No other statistically significant differences were observed, but the power of the sample size was low.