Chronic mountain sickness (CMS) is characterized by excessive erythrocytosis (EE) secondary to hypoventilation. Erythropoietin (Epo) and testosterone regulate erythrocyte production. Low thyroid hormone levels are also associated to hypoventilation. Hence, it is possible that these hormones can play a role in etiopathogeny of EE. The purpose of this study was to elucidate the effect of sexual and thyroid hormones and erythropoietin in residents from Lima (150 m) and Cerro de Pasco (4,340 m) and the response to human chorionic gonadotrophin stimulation (hCG) of the hormone(s) associated to EE. Three groups were studied, first at low altitude (150 m) and second at high altitude (one with haemoglobin values >16-21 g/dl and the second with Hb>21 gr/dl (EE)). hCG was administered intramuscularly in a single dose (1000 IU) and blood samples obtained at 0, 6, 12, 24, 48 and 72 hours after injection. High altitude natives present similar levels of gonadotrophins and thyroid hormones, but lower dehydroepiandrosterone sulphate (DHEAS) levels (113.92±29.6 ug/dl vs 213.95±18.18 ug/dl; mean±standard error of the mean; P<0.01) and greater erythropoietin (34.70±5.08 mIU/ml vs 12.63±0.97 mIU/ml, P<0.01), 17-hydroxyprogesterone (0.88±0.08 vs 0.27±0.07 ng/ml; P<0.01) and testosterone levels (428.52±41.2 vs 199.09±47.46 ng/dl; P<0.01) than those at 150 m. Serum testosterone levels (524.13±55.91 ug/dl vs 328.14±53.23 ng/dl; P<0.05) and testosterone/DHEAS ratios are higher (7.98±1.1 vs 3.65±1.1; P<0.01) and DHEAS levels are lower in the EE group (83.85±14.60 ug/dl vs 148.95±19.11 ug/dl; P<0.05) whereas Epo was not further affected. Testosterone levels were highest and DHEAS levels were lowest in the EE group at all times after hCG stimulation. In conclusion, the study demonstrated that high androgen activity could be involved in the etiopathogeny of the CMS. These evidences provide an opportunity to develop new therapeutic strategies.