Bilateral wedge biopsies were practiced in 22 boys, aged 4 to 13 years, with Acute Lymphoblastic Leukemia after 36 months in continuous remission. All the patients had been given the same chemotherapy (protocol D.74). The aim of this study was to detect the presence of residual leukemic infiltrates before suppressing therapy and to evaluate gonadal toxicity due to chemotherapy. Leukemic infiltration was found in six cases (27%) and another two were considered as doubtful cases. Interstitial space and subepithelial areas were the preferred sites of infiltration; tunica albuginea was found affected in only two cases. All patients affected were given local radiotherapy and additional chemotherapy. Interstitial oedema with fibroblasts and macrophages was the most common lesion (81%); fibrosis was observed in 20% of the cases. Changes in testicular morphology were mainly reflected by narrowing of the tubular diameter (less than 75% of average values in 63% of the cases) and decrease of the number of spermatogenic cells per 100 tubules (less than 75% of mean values in 86% of the biopsies). These changes appeared as independent of the presence of leukemic infiltrates. Conclusions. Biopsies performed in boys before suppression of therapy are considered as important; in positive cases additional local and systemic therapy must be given. Further longitudinal and functional studies are needed to verify consequences of toxic effects observed.