This study investigated reproductive hormones changes, body composition and semen in seven male trekkers. Resting levels of FSH, LH, T, E2, PRL and cortisol, were evaluated at sea level (SL), after 43 days of altitude trekking (0–5800) (AT), 10 days (10D) and six months after returning at SL (6M). BMI, waist circumference, % fat, and semen analysis, were evaluated. Compared to SL, FSH and LH were higher at AH (p<.0002 and p=0.03) and decreased at 10D, returning to baseline at 6M. PRL and cortisol showed the same trend of the two gonadotropins. E2 showed lower levels at AT (p=0.001) compared to SL and progressively increased returning to baseline level at 6M. T level was unchanged at AT compared to SL, while levels were higher at 10D (p<.0002), and returned to baseline at 6M. BMI, waist circumference and % fat were reduced (p=.04, p=.01, p=.001, respectively) at 10D compared to SL, and returned to baseline at 6M. T level was strictly negatively correlated with BMI (p=0.02; r=-0.81), waist circumference (p=0.05; r=-0.75) and %fat (p=0.04; r=-0.78) at 10D. A reduction of volume and sperm concentration (p=0.05), with unchanged sperm forward motility, morphology, and DNA fragmentation were observed at 10D, and normalyzed at 6M. Hypoxia during altitude trekking was associated to increased levels of T, probably related to changes in body composition. This effect was shadowed, just after trekking, by a possible negative testicular effect of hypoxia and physical endurance.