The adaptations to high altitudes or hypoxia have mixed effects on the maximal oxygen uptake VO(subscript 2max) at sea level. This study investigated the change of VO(subscript 2max), and the possible hematological and ventilatory adaptation mechanisms, after returning from a 17-day expedition to Peak Lenin (7134m). Four mountaineers (three males and one female) were recruited. Hematological parameters, serum erythropoietin (EPO), VO(subscript 2max), and maximal ventilation were measured by Bruce protocol at baseline (20 days before arriving at the base camp) and eight days after their return. Serum EPO was also measured immediately after returning from the summit. The results were shown that absolute VO2(subscript) max did not change, while VO(subscript) 2max relative to body weight, maximal ventilation, and ventilation threshold increased significantly (p＜.05) after the expedition. Serum EPO marginally increased by 36.52±22.36% (p=0.053) during the expedition, then significantly decreased by 39.78±5.78% (p＜.05) after return to sea level. Hematocrit and mean corpuscular volume (MCV) significantly elevated, while mean corpuscular hemoglobin concentration (MCHC) significantly (p＜.05) decreased after the expedition. Hemoglobin was shown a trend of increase (15.0±1.6 vs 15.8±1.5g/dl, p=0.085) after the expedition. The two subjects have higher serum concentration in EPO than others. It showed the elevation in hemoglobin levels and VO(subscript 2max). This study suggested that relative VO(subscript 2max) increased after returning from a high-altitude expedition. The increase in VO(subscript 2max) at least partially resulted from hematological and ventilatory adaptations.