Between August, 1978, and March, 1981, 24 boys with acute lymphocytic leukaemia (ALL) (14 non-B-cell non-T-cell, 6 pre-B-cell, and 4 T-cell) underwent bilateral wedge testicular biopsy at initial diagnosis. All testes were of normal size. Histological analysis was performed independently by three pathologists, 4 (3 non-B non-T, and 1 pre-B) of 20 patients with non-T-cell ALL demonstrated testicular focal lymphoblastic involvement. Successful induction chemotherapy without concomitant testicular irradiation completely eradicated testicular infiltrates in all 4 patients. Only 1 of the 4 T-cell patients had pre-treatment testicular infiltrates. Unlike boys with non-B non-T leukaemia, attainment of bone-marrow remission in this patient was associated with persistent testicular disease. These preliminary data indicate that in patients with ALL of non-T-cell type, systemic induction chemotherapy eradicates the small testicular infiltrates of lymphoblasts. Prophylactic testicular irradiation in this group of patients does not seem to be warranted.