Wedge biopsy of the testis was performed in 46 children who had received long-term chemotherapy for acute lymphoblastic leukemia. Occult testicular infiltration was noted in three children (6.5%). Two of three children with biopsy-proven infiltration died of systemic disease in spite of local irradiation and reinduction chemotherapy. Six of 43 children shown to be negative by testicular biopsy relapsed 11 months to 15 years later, and 3 of 6 patients died of systemic disease, but none of the cases developed testicular disease. Chemotherapy-induced gonadal damage was observed in 30 of 46 children, and tubular damage was occasionally still seen 4 years after cessation of treatment. Although gonadal damage usually depends on the cumulative dosage of cyclophosphamide, intact tubular fertility index was found in several children who had received a greater dose of cyclophosphamide intermittently. Induction and maintenance chemotherapy for acute lymphoblastic leukemia had little influence on hormonal function. Testicular biopsy at the time of cessation of chemotherapy seems to be worthwhile for the subsequent strategy of treatment, and long-term surveillance for gonadal damage of long-term survivors will be required.