The clinical value of early detection of testicular leukaemia was assessed by elective wedge biopsy during the 12th or 18th month of therapy in children with acute lymphoblastic leukaemia. Biopsy samples were taken in 106 of 238 consecutively treated boys who attained complete remission; only 1 had occult disease. Samples were also taken in 2 boys with testicular leukaemia at diagnosis as soon as they achieved complete remission, and in 14 others at the end of therapy; none showed evidence of leukaemic infiltration. 13 boys, including the 1 with biopsy-confirmed occult disease, had a testicular relapse; 6 of these patients had had negative biopsy findings 12-28 months before relapse. These results indicate that elective testicular biopsy during therapy for childhood leukaemia is of no benefit clinically. They also suggest that routine use of end-of-therapy testicular biopsy should be re-evaluated, since frequency of testicular relapse is low, therapy is effective, and negative biopsy findings do not preclude eventual relapse.