Hypothalamo-hypophyseal-testicular function was studied in twenty-eight prepubertal boys with ALL in clinical and haematological remission. Eighteen were treated with combined systemic chemotherapy (24-36 months) and the other ten, who had testicular leukemic infiltrates, received chemotherapy (38-60 months) and testicular radiotherapy (2 000 rad). Plasma levels of LH and FSH were measured before and after stimulation with LHRH (100 micrograms i.v.) and plasma levels of testosterone before and after stimulation with hCG (1 500 IU/48 h/7 doses). In patients treated with chemotherapy alone, mean basal LH and FSH, mean responses to LHRH stimulation and mean testosterone levels after stimulation with hCG did not significantly differ from those of the controls. Five of these patients who had normal testosterone values after three doses of hCG had testosterone values below the normal range after seven doses. In patients treated with chemotherapy and testicular radiotherapy, mean basal FSH and mean responses to LHRH stimulation were significantly higher than those of the controls. Testosterone values after stimulation with hCG were low in three and very low in the other seven. In both groups of patients data from testicular biopsies were consistent with functional results. We conclude that chemotherapy causes slight testicular damage, but chemotherapy and testicular radiotherapy produce severe testicular damage in patients with testicular leukemic infiltrates.