The testicular function of 30 adolescent or adult males having undergone polychemotherapy in childhood was assessed by means of a spermogram or testicular biopsy. At the time of examination, the patients were pubertal and had completed chemotherapy between 1 and 20 years previously (mean, 9 years). All patients who were prepubertal or intrapubertal at the time of treatment achieved normal puberty with normal growth. Twenty patients presented with azoospermia and/or severe disturbances in the germinal line on biopsy. This series confirms the toxicity of alkylating agents, in particular that of the mechlorethamine, vincristine, procarbazine, and prednisone combination (MOPP) and that of cyclophosphamide (CPM). However, dactinomycin, vinblastine, and vincristine did not appear to have a toxic effect on spermatogenesis. The prepubertal state did not protect the gonads of 19 patients who were prepubertal at diagnosis: 12 are now sterile as a result of the treatment. An increase in basal follicle-stimulating hormone (FSH) levels gives a good indication of testicular damage, although normal levels do not rule out the possibility of azoospermia.