All male renal transplant recipients with a functioning allograft 1 month post transplant will be eligible to participate in this prospective serial observation study. As roughly half of renal transplant patients are assigned to rapamycin immunnosuppression clinically and half are not, we plan to assess routine clinical perameters, sex hormone levels, subjective sexual function as measured by standardized questionnaire and semen analysis in 55 patients on sirolimus treatment and 55 patients not on this treatment. Assessment will be made at baseline (1 mo post transplant) and 6 and 12 months later. All adult male renal transplant recipients ages 21-65 off dialysis 1 mo post transplant will be eleigible. Exclusion criteria will be female gender, patients requiring testosterone therapy, patients on dialysis, and those unwilling to give informed consent.