The Tibetan population, long a resident on the Qinghai-Tibetan Plateau, has lower hemoglobin concentrations than Han Chinese migrants, but it is incompletely known how gender effects the hemoglobin concentrations in the two populations at various altitudes. Measurements of hemoglobin concentration were obtained in 5887 healthy male and female Tibetan and Han residents aged 5-60 years, at altitudes of 2664, 3813, 4525, and 5200 m. Multiple regression equations showed the beta-coefficients for altitude and for age were higher (p<0.05) in Han males than in Tibetan males and in Han females than in Tibetan females. Analysis indicated a significant three-way interaction between altitude, gender, and ethnicity (X²=3.72, p=0.05). With increasing altitude, males progressively had more hemoglobin than females in the Han, but not the Tibetan population. Above 2664m this gender-related difference in hemoglobin concentration increased from childhood to young adulthood more in Han than in Tibetans. We suggest that the Han - Tibetan ethnic difference in the effect of altitude on hemoglobin concentration depends to a large extent on gender.