The 1911 Anglo-American Expedition to Pikes Peak was the most important high altitude expedition in the early 20th century. By way of introduction, it is useful to look at the state of high altitude physiology when the expedition was being organized. A watershed event had been the publication in 1878 of La Pression Barométrique by the eminent French physiologist, Paul Bert. He showed conclusively that the deleterious effects of high altitude could be attributed to the low partial pressure of oxygen. Previous theories had included bizarre explanations such as the failure of the reduced barometric pressure to press the head of the femur into the pelvis with the result that the required increased muscular effort caused fatigue. However Bert showed that the low PO2¬ was the culprit. Bert never carried out studies at high altitude and all his work was done in decompression chambers in the Sorbonne in Paris. However shortly after the publication of his book physiologists recognized the advantages of working at high altitude. In 1890 the first high altitude laboratory known as the Observatoire Vallot, was installed at an altitude of 4350 m on Mont Blanc. This was followed in 1893 by the Capanna Margherita which was erected on a peak of the Monte Rosa in Italy at an altitude of 4559 m as a result of the enthusiasm of the Italian physiologist Angelo Mosso. There was also an expedition to the Alta Vista Hut, altitude 3350 m in Tenerife, Canary Islands, in 1910. The principal figures of the Pikes Peak expedition were J.S. Haldane and C.G. Douglas from Oxford, Y. Henderson from Yale, and E.C. Schneider from Colorado Springs. The choice of Pikes Peak for the venue was largely determined by the difficult access of the stations on Mont Blanc and Monte Rosa and the primitive living conditions there. At the outset of planning the expedition, Haldane stated that he was looking for a “nice comfortable mountain” and, as Henderson related, Haldane thought that the rigors of working in the Capanna Margherita made it difficult to distinguish between the effects of the bad food and those of the high altitude. The same sentiment was aired by Joseph Barcroft who complained of the difficulties of gastronomy. One wonders if the British physiologists had the Right Stuff. However Pikes Peak had many advantages including its substantial altitude of 4300 m, easy access via a cog railway, comfortable, spacious living accommodation on the summit, and access to the nearby laboratory at Colorado College if additional equipment or supplies were needed. The classical design of the expedition included prior measurements at sea level in Oxford or New Haven, rapid ascent to 4300 m where the participants remained for five weeks, and then a rapid descent to sea level where further studies were made. An important feature was that many studies were made during the deacclimatization period. Few high altitude expeditions have included these. The extensive scientific program included descriptions of acute mountain sickness, multiple measurements of alveolar gas and blood partial pressures before, during, and after the period at high altitude, changes of ventilation including the marked hyperpnea and periodic breathing, exercise studies, studies of blood hemoglobin and plasma volume, and limited cardiovascular measurements,. A major interest was the relations between alveolar and arterial PO2, and the participants concluded that active secretion of oxygen by the lung was a feature of acclimatization although this was subsequently shown to be erroneous. The arterial PO2 was determined by an indirect method following carbon monoxide breathing and the precise reason for the errors is still not known. A colorful participant was Mabel FitzGerald who had collaborated with Haldane in Oxford several years previously. However she did not take part in the summit studies for reasons that are not completely clear. It has been alleged that this was because she was not chaperoned, but it is more likely that her presence would have complicated the living conditions. Instead of participating in the measurements on the summit she visited various Colorado mining camps accompanied only by a mule. In the process she collected extensive data on alveolar gas at different altitudes, and it is remarkable that her results are frequently cited even today because so few measurements have been made at relatively low altitudes. Mabel FitzGerald was able to join the Haldane centenary celebration in Oxford in 1961 at the age of 100 when she was given an MA degree and made a member of the Physiological Society. The Pikes Peak Expedition remains one of the most important, and its influence on subsequent high altitude studies was immense.