We investigated how acclimatization effects achieved during a high-altitude alpinist expedition influence endurance performance, ventilation ( V ˙ E ) and blood oxygen saturation (SaO₂) in normoxic (NOR) and hypoxic conditions (HYP). An incremental testing protocol on a cycle ergometer was used to determine the power output corresponding to the Lactate (P_LT) and Ventilatory Threshold (P_VT) in NOR and HYP (FiO₂=0.13) as indirect characteristics of endurance performance in both conditions. Furthermore, changes in V ˙ E , SaO₂, blood pH and P_co2 were measured at a similar absolute exercise intensity of 180 W in NOR and HYP conditions. Seven experienced alpinists (mean ± SD: age: 50 ± 6 yrs; body mass: 76 ± 5 kg; body height: 175 ± 8 cm) volunteered to participate in this study after they had reached the summit of Gasherbrum II and Ama Dablam. They had therefore experienced the limitations of their acclimatization. Individual differences of P_LT between values reached after and before the expedition (∆P_LT) correlated (r = 0.98, p = 0.01) with differences of SaO₂ (∆SaO₂) in HYP, and differences of P_VT (∆P_VT) correlated (r = -0.83, p = 0.02) with differences of V ˙ E ( Δ V ˙ E ) in HYP. The results suggest that the acclimatization may not have an equivocal and simple influence on the performance in hypoxia: enhanced blood oxygen saturation may be accompanied by increased endurance only, when the increase exceeded 2-3%, but enhanced ventilation, when increased more than 10 l/min in HYP, could detrimentally influence endurance.