The appearance of seminiferous tubules and interstitial cells of children, aged 2.5 to 13 years, affected by acute lymphoblastic leukemia was analyzed in sections. The testicular biopsies were performed at the end of therapy (vincristine, prednisone, L-asparaginase, 6-mercaptopurine, intrathecal methotrexate), which was affected for the same period and at the same doses. Three age groups were considered (I, 2.5 to 5 years; II, 6 to 9 years; III, 12 to 13 years). Age groups I and II presented damage of some tubules (25–35%) and areas of degeneration. Histometric analysis performed for A type spermatogonial population gave a mean value corresponding to controls in age group I and a mean value significantly lower with respect to controls in age group II. Moreover, age group II presented a lack of increase in tubular cross section. These results suggest that there is a vulnerability both of whole tubules and of some areas of Sertoli cells and germ cells to cytotoxic-induced damage. Leydig cells appear to be the cells least sensitive to drugs, and hormonal data indicate that the hypothalamic pituitary function appears to be intact, despite chemotherapy. Long-term prospective studies of reproductive function in children receiving cancer chemotherapy are needed to determine the magnitude and duration of damage resulting from therapeutic treatment.