Gonad function and sexual activity were evaluated in 44 young male adults who received a kidney shortly before or during adolescence. All patients were growth-retarded (-2.2 SD). Volume of the testes was decreased less than 12 ml, i.e, less than 10th centile) in 47% of patients, whereas hormone levels (testosterone, FSH, LH, and prolactin) were within the normal range. Oligospermia or azoospermia was found in 10 of 11 tested patients. Testicular function and sperm count were negatively correlated with renal function. Age at first ejaculation was increased and 57% of patients had never had sexual intercourse, although all patients reported normal sexual drive and normal erections.