Thirty men treated in childhood with cyclophosphamide for a mean of 280 days were assessed at a mean of 12.8 years after treatment for hormone concentrations and spermatogenesis. Four were azoospermic, nine oligospermic, and 17 normospermic. There was a significant inverse correlation of sperm density with cyclophosphamide dosage and duration of treatment. After a further mean follow up of 7.2 years three patients who were previously oligospermic and one who was azoospermic had normal sperm counts. All patients had normal sexual characteristics and libido. Serum androgen and prolactin concentrations did not differ significantly between patients and controls. Raised basal and stimulated follicle stimulating hormone concentrations were in keeping with impaired spermatogenesis. All patients had significantly raised luteinising hormone responses on stimulation with luteinising hormone releasing hormone. The results suggest compensated Leydig cell failure, and patients with this condition require long term evaluation of testicular function. Potential recovery of spermatogenesis with time requires appropriate counselling and contraceptive advice.