A total of 614 healthy western trekkers (487 completed the trial) were assigned to receive Ginkgo biloba, acetazolamide, combined acetazolamide and G. biloba, or placebo, initially taking at least three or four doses before continued ascent of Mount Everest. The incidence of acute mountain sickness was measured by Lake Louise acute mountain sickness score = 3 with headache and one other symptom. Secondary outcome measures included blood oxygen content, severity of syndrome (Lake Louise scores = 5), incidence of headache and severity of headache. Ginkgo biloba was not significantly different from placebo for any outcome; however, participants in the acetazolamide group showed significant levels of protection. The incidence of acute mountain sickness was 34% for placebo, 12% for acetazolamide, 35% for G. biloba and 14% for combined G. biloba and acetazolamide. The proportion of patients with increased severity of acute mountain sickness was 18% for placebo, 3% for acetazolamide, 18% for G. biloba and 7% for combined G. biloba and acetazolamide.