Objective: To evaluate an ambulatory physiological monitoring system during a mountaineering expedition, We hypothesized that the Environmental Symptoms Questionnaire, combined with frequent measurement of oxygen saturation and core temperature, would accurately identify cases of environmental illness. Methods: Twelve military mountaineers took a daily Environmental Symptoms Questionnaire, monitored fingertip oxygen saturations, and recorded core temperatures while climbing a 4,949-m peak. illnesses identified by the system were compared with those identified by spontaneous reports. Results: The system correctly identified one case of high-altitude pulmonary edema and two illnesses that were not reported to the physician (one case of acute mountain sickness and one of self-limited symptomatic desaturation), However, it did not identify two illnesses that were severe enough to preclude further climbing (one case of sinus headache and one of generalized fatigue). Conclusions: Our monitoring system may complement, but cannot replace, on-site medical personnel during mountaineering expeditions.