BACKGROUND: Time of useful consciousness is related with various factors including smoking, age, sex, drug, and temperature. It is still unclear whether acute hypoxia tolerance is different between male and female. METHOD: We included 32 healthy students (male, n=16; female, n=16) volunteered to participate in this study. The blood hemoglobin concentration was measured at a day before flight. With high altitude rapid decompression flight training chamber, flight to simulated altitude of 25,000 ft was performed after nitrogen extraction breath. At 25,000 ft, heart rate and arterial O2 saturation (SaO2) were measured by using pulse oximeter after taking off oxygen mask. We compared the duration from mask-off to the time at 60% SaO2 between male and female and correlated the time at 60% SaO2 with hemoglobin concentration. RESULTS: The duration from mask-off to SaO2 reaching 60% were significantly longer in the male group. There was a linear relationship between hemoglobin concentration and time at 60% SaO2. CONCLUSION: Acute hypoxia tolerance differs from the two sexes and the difference of hemoglobin concentration is one of possible causative factors of this difference.