An acute increase in hemoglobin concentration ([Hb]) up to 1.6 g/dL has been previously reported under the condition of exercise-induced hypoxemia in sea level individuals with COPD (Schagatay et.al 2015). Changes in [Hb] have been attributed to splenic contraction. To determine whether, and to what extent, [Hb] changes acutely in response to exercise in long-term residents at high altitude, we compared hemoglobin concentration at rest and during exercise in male Andean (n = 14; 4330 m) and Tibetan (n = 21; 4200 m) native highlanders, whose ancestors have resided for hundreds of generations at altitude, as well as Han Chinese (n = 9; 4200 m) high-altitude residents. In the latter two populations, we further examined male (Tibetan n = 10; Han n = 8) and female (Tibetan n = 9; Han n = 10) individuals both at a resident altitude of 2200 m and simulated altitude of 4200 m in a hypobaric chamber. We measured [Hb] at rest ([Hb]r), and peak exercise ([Hb]p), as well as blood oxygen saturation (SaO2), cardiac output (QT), VO2 and body mass index (BMI) and compared [Hb]p - [Hb]r, denoted as ?[Hb], for male Andeans at 4300 m, male Tibetans and Han Chinese resident at 4200 m, male and female Tibetans and Han Chinese resident at 2200 m and under simulated 4200 m. [Hb] significantly increased with exercise in all groups (P < 0.001 each) except for Han Chinese and Tibetan females at 4200 m. For male Andeans at 4300 m, ?[Hb] was 1.11 g/dL. For Han Chinese and Tibetans at 2200 m, ?[Hb] was 1.09 g/dL and 0.94 g/dL, respectively (males), and 1.17 g/dL and 1.01 g/dL respectively (females). For Han Chinese and Tibetans at 4200 m, ?[Hb] was also significant (p < 0.001) at 1.20 g/dL and 0.87 g/dL respectively in males, but in females the increases were not. No significant difference of ?[Hb] is observed across different ethnic groups regardless of sex at high altitude. The correlation between ?[Hb] and SaO2 at maximum exercise level is found to be positive in the Andeans (p = 0.007) and not detected in Tibetans or Han Chinese. In addition, there is no significant correlation found between ?[Hb] and BMI, [Hb]r, QTmax or VO2max. In conclusion, we observed an exercise-induced increased [Hb] among Andeans, Tibetans and Han Chinese at high altitude and Tibetans and Han Chinese only at intermediate altitude. Thus, measurement of [Hb] in-situ during hypoxia/exercise experiments are important to incorporate in studies of O2 transport. Furthermore, larger ?[Hb] can be potentially beneficial to maintaining higher oxygen saturation at high altitude during acute exercise. [Hb](g/dL) of Tibetans, Han Chinese and Andeans Ancestry Tibetans Han Chinese Andeans Exercise Level Rest Peak Rest Peak Rest Peak Male 15.4 ± 1.3 16.2 ± 1.2 16.7 ± 1.3 17.4 ± 1.1 2200 m Female 12.9 ± 1.8 13.5 ± 1.8 13.4 ± 1.2 14.1 ± 1.4 Male 15.1 ± 1.0 16.1 ± 1.4 16.2 ± 1.0 17.1 ± 0.9 17.3 ± 1.6 18.4 ± 1.7 4200 m Female 12.7 ± 1.5 13.1 ± 1.4 13.2 ± 1.3 13.8 ± 1.2