Basu M, Malhotra AS, Pal K, Prasad R, Kumar R, Prasad BAK, Sawhney RC. Erythropoietin levels in lowlanders and high-altitude natives at 3450 m. Aviat Space Environ Med 2007; 78:963–7. Background: This study is aimed to determine whether short or prolonged residency at high altitude (HA) elicits erythropoietin (EPO) secretion effectively in subjects who were able to acclimatize and those who were not able to acclimatize and suffered from acute mountain sickness (AMS) and high altitude pulmonary edema (HAPE). Methods: Plasma EPO was measured in 16 lowland residents (LLR) at sea level (SL) and during 11 d of their sojourn at an altitude of 3450 m. Identical studies were also conducted in LLR acclimatized to HA (LLR-accl), high altitude natives (HAN) and in patients of AMS and HAPE. Results: In LLR at SL, the mean ± SD EPO levels were 8.93 ± 3.75 mU · ml⁻¹, increased significantly after 8 h (20.0 ± 11.06) of arrival at HA, peaked by day 1 (27.91 ± 10.74 mU · ml⁻¹), and started declining thereafter. The hemoglobin and hematocrit also increased after 8 h of arrival at HA and the increased levels were maintained during sojourn at high altitude. The EPO levels in LLR-accl were found to be significantly higher than the LLR SL values, but were not significantly different in HAN. The EPO levels in patients of AMS were not significantly different than the LLR values during the initial 2 d after arrival at HA but were found to be increased in patients of HAPE. Conclusion: Short or prolonged residency at HA is associated with increased secretion of EPO. The EPO response to hypoxia is not significantly altered in AMS but is markedly enhanced in HAPE, which may be due to exaggerated hypoxemia in these patients.