The aim of this study was to analyze the semen variables and hormone profiles in kidney transplanted male adolescents. Eight post-pubertal male patients who underwent successful renal tx during the peripubertal period and who had ESRD during childhood were enrolled in the study. Patients who underwent tx before 14 yr old (group I) and patients who underwent tx after 14 yr old (group II) were evaluated separately. Semen was collected and analyzed. Serum levels of LH, FSH, and testosterone were measured and found to be normal in all patients except one. The mean age at the diagnosis of CKD was six yr and 13 yr in groups I and II, respectively. The mean age at the time of tx was 12 yr in the first and 17.8 yr in the second group. The patients in group I had received prednisone, cyclosporine A and azathioprine with a longer duration of time compared with patients in group II. Sperm counts (15.5 +/- 15.7 vs. 82.3 +/- 64.2 millions/mL) and sperm motilities (37.8 +/- 30.9 vs. 57.8 +/- 22.1%) were lower in group I than group II. Only one patient in group II had normal sperm parameters and azospermia was observed in one patient from group I. We conclude that the earlier onset and the longer duration of uremia, the more impairment of reproductive function. Also, it seems that duration of exposure to corticosteroids or cyclosporine combined with azathioprine contribute to sperm dysfunction in peripubertal transplanted boys.