Thirteen out of 163 male children with acute leukemia developed extramedullary involvement of the testes. This represented an incidence of 8.0%. These 13 children experienced 19 separate episodes of enlargement. All episodes were treated with radiotherapy. The enlargement was unilateral in 9 episodes and bilateral in 10. The bone marrow (BM) was in complete remission in 13 episodes. In 8 of 13 patients the testis was the first site of relapse since the initial BM remission. Of the males with a WBC less than 20,000/mm3 on initial diagnosis of leukemia, 4.3% developed this testicular complication, as compared to 17.0% of those with a WBC equal to or greater than 20,000/mm3. The median time interval to testicular involvement was 13 months; to death, 9 months. Relapse at other sites usually followed testicular involvement. Autopsy findings did not appear to be dose-related. Radiotherapy was clinically successful when a total dose in the range of 1200 rads was used, and is the treatment of choice for this complication.