INTRODUCTION: Dramatic increases in parachuting safety over the last three decades have been attributed to advances in technology and training for parachutists. However, very little is known about the physiological condition of skydivers making repeated, medium-altitude aircraft exits without using supplemental oxygen. As in aviation, human error is broadly responsible for the majority of skydiving mishaps, although it is unclear what role, if any, physiological factors contribute to these mishaps. Over the course of 2 d, a healthy, 50-yr-old male skydiver executed four normal exits (two jumps per day) from an aircraft between 13,500 and 14,000 ft (4115 and 4267 m) pressure altitude while wearing a helmet-mounted biomonitoring device (SPYDR, Spotlight Labs). On both days, after the subject's second jump, he reported feeling lightheaded and dizzy, symptoms he experiences approximately once every five jumps, and had previously attributed to the excitement of the jump. Inspection of Spo₂ and pulse data revealed that the subject was mildly hypoxic at jump altitude (Spo₂ < 90%). For all four jumps, Spo₂ did not return to normal levels until under canopy. Previous studies have evaluated the cognitive impairment of general aviation pilots operating unpressurized aircraft above 12,500 ft (3810 m) without supplemental oxygen. Alarmingly, mildly hypoxic pilots exhibited twice the rate of procedural errors as compared to normally oxygenated subjects. This study found that the skydiver exited the aircraft with mild hypoxia, which has been associated with cognitive impairment in pilots and could possibly be linked to injuries and/or fatalities.Bradke BS, Everman BR. Mild hypoxia of a skydiver making repeated, medium-altitude aircraft exits. Aerosp Med Hum Perform. 2020; 91(2):110-115.