Aim: To evaluate the effect of chronic hypoxia on human spermatogenic parameters and their recovery time. Methods: Seminological parameters of six male healthy mountain trekkers were evaluated in normoxia at sea level. After 26 days exposure to altitude (ranging from 2 000 m to 5 600 m, Karakorum Expedition) the same parameters were again evaluated after returning to sea level. These parameters were once again evaluated after 1 month and then again after 6 months. Results: Sperm count was found to be lower immediately after returning to sea level (P = 0.0004) and again after a month (P = 0.0008). Normal levels were reached after 6 months. Spermatic motility (%) shows no reduction immediately after returning to sea level (P = 0.0583), whereas after 1 month this reduction was significant (P = 0.0066). After 6 months there was a recovery to pre-hypoxic exposure values. Abnormal or immature sperma- tozoa (%) increased immediately after returning to sea level (P = 0.0067) and then again after 1 month (P=0.0004). After 6 months there was a complete recovery to initial values. The total number of motile sperm in the ejaculate was found to be lower immediately after returning to sea level (P = 0.0024) and then again after 1 month (P = 0.0021). After 6 months there was a recovery to pre-hypoxic exposure values. Conclusion: Chronic hypoxia induces a state of oligospermia and the normalization of such seminological parameters at the restoration of previous normoxic conditions after 6 months indicate the influence of oxygen supply in physiological mechanisms of spermatogenesis and male fertility. (Asian J Androl 2008 Jul; 10: 602-606)