The clinical courses of four boys with acute lymphocytic leukemia who developed leukemic infiltration of their testes while in bone marrow remission are described. Clinically, they had only unilateral involvement; however, histologically, bilateral disease was present in the three patients who underwent biopsy of the contralateral organ. The development of testicular leukemia did not herald immediate bone marrow relapse. Management is discussed which includes surgery and radiotherapy.