Gonadal function was assessed in seventeen adult male renal transplant recipients, with well established good homograft function, for a mean of 4.9 years. Patients were assessed clinically and by measurement of basal concentrations of FSH, LH, prolactin, testosterone and oestradiol, FSH and LH responses to bolus injections of LHRH and semen analysis. Retrospectively all had symptoms consistent with marked hypogonadism prior to transplantation but in nine out of sixteen this was reversed with transplantation. Residual hypogonadism was evident in seven of sixteen patients and correlated with duration of haemodialysis longer than 1 year (P less than 0.01). Even among patients with clinically normal gonadal function, defects in the hypothalamic--pituitary--testicular axis remained. Elevated basal serum FSH, excessive FSH responses to LHRH and lowered basal serum testosterone were found. In the group with residual hypogonadism more marked changes, including elevated basal LH and excessive LH responses to LHRH, were also found. Fertility was recorded in two men on three occasions since transplantation. Sperm counts were normal in five and abnormal in four patients. Testicular volume and sperm density were inversely correlated with basal and stimulated FSH and LH levels.