Hypobaric hypoxemia is a well-known risk of aeromedical evacuation (AE). Validating patients as safe to fly includes assessment of oxygenation status as well as oxygen carrying capability (hemoglobin). The incidence and severity of hypoxemia during AE of non-critically injured casualties have not been studied. For this study, subjects deemed safe to fly by the validating flight surgeon were monitored with pulse oximetry from the flight line until arrival at definitive care. All subjects were U.S. military personnel or contractors following traumatic injuries. Non-invasive oxygen saturation (SpO2), pulse rate, and non-invasive hemoglobin were measured every 5 seconds and recorded to electronic memory. Patient demographics and physiologic data were collected by chart abstraction from Air Force Form 3899, patient movement record. The incidence and duration of hypoxemic events (SpO2 90%) and critical hypoxemic events were determined (SpO2 85%). Sixty-one casualties were evaluated during AE from Bagram Air Base to Landstuhl Regional Medical Center. Mean age was 26.2 6 years, Injury Severity Score was 8 11, and mean SpO2 prior to AE was 96 2%. The mean transport time was 9.3 1.3 hours. Patients were monitored prior to AE for a brief period, yielding a total recording time of 10.28 hours. The mean hemoglobin at the time of enrollment was 13.2 3.5 g/dL (9.4-18.0g/dL). Hypoxemia (SpO2 90%) was seen in 55/61 (90%) of subjects. The mean duration of SpO2 90% was 44 minutes. The mean change in SpO2 from baseline to mean inflight SpO2 was 4 1.2%. Thirty-four patients (56%) exhibited an SpO2 85% for 11.7 15 minutes. In conclusion, hypoxemia is a common event during AE of casualties. In patients with infection and concussion or mild traumatic brain injury, this could have long-term consequences.