Life at high altitudes (>4000m) is associated with higher erythropoiesis. Haemoglobin ≥21gdl-1 is considered as excessive erythrocytosis and is a sign of chronic mountain sickness (CMS). The present study was designed to determine an association between serum testosterone (T) and serum oestradiol (E2) levels with the score of CMS. One hundred and seventeen men natives from low altitude (150m) and 103 men natives from high altitude (4340m) were studied. The presence of breathlessness or palpitations, sleep disturbance, cyanosis, dilatation of veins, paraesthesia, headaches, tinnitus and Hb ≥21gdl-1, have been included for the CMS score. Men living at high altitude had higher CMS score (P<0.001), serum T (P<0.05) and serum E2 levels (P<0.04) and had lower serum luteinising hormone levels (P<0.005) than men living at sea level. At high altitude, the group with the highest CMS score (≥10) showed higher chronological age, SpO2, serum T and ratio T/E2 than the group with CMS score of ≤4. Some symptoms of CMS as sleep disorders and paraesthesia were more related to high serum T level; cyanosis was more related to higher haemoglobin values. In conclusion, higher serum T levels were associated to higher scores of CMS. © 2011 Blackwell Verlag GmbH.