Alexander M. Kellas (1868-1921) was a British physiologist who made pioneering contributions to the exploration of Everest and to the early physiology of extreme altitudes, but his physiological contributions have been almost completely overlooked. Although he had a full-time faculty position at the Middlesex Hospital Medical School in London, he was able to make eight expeditions to the Himalayas in the first two decades of the century, and by 1919 when the first official expedition to Everest was being planned, he probably knew more about the approaches than anybody else. But his most interesting contributions were made in an unpublished manuscript written in 1920 and entitled "A consideration of the possibility of ascending Mount Everest." In this he discussed the physiology of acclimatization and most of the important variables including the summit altitude and barometric pressure, and the alveolar PO2, arterial oxygen saturation, maximal oxygen consumption, and maximal ascent rate near the summit. On the basis of this extensive analysis, he concluded that "Mount Everest could be ascended by a man of excellent physical and mental constitution in first-rate training, without adventitious aids [supplementary oxygen] if the physical difficulties of the mountain are not too great." Kellas was one of the first physiologists to study extreme altitude, and he deserves to be better known.