The effect of testicular irradiation on Leydig cell function has been studied in a group of boys irradiated between 1 and 5 years earlier for a testicular relapse of acute lymphoblastic leukaemia. Six of the seven boys irradiated during prepubertal life had an absent testosterone response to HCG stimulation. Two of the four boys irradiated during puberty had an appropriate basal testosterone level, but the testosterone response to HCG stimulation was subnormal in three of the four. Abnormalities in gonadotrophin secretion consistent with testicular damage were noted in nine of the 11 boys. Evidence of severe Leydig cell damage was present irrespective of whether the boys were studied within 1 year or between 3 and 5 years after irradiation, suggesting that recovery is unlikely. Androgen replacement therapy has been started in four boys and will be required by the majority of the remainder to undergo normal pubertal development.