Seven children with acute lymphocytic leukemia developed testicular swelling during bone marrow remission. Needle biopsies and one orchiectomy specimen revealed leukemic infiltration. Another patient encountered concurrent medullary and testicular relapses. The leukemic nature of the testicular infiltration can be readily confirmed on needle biopsies. Orchiectomy should be discouraged. The leukemic infiltration mainly took place in the interstitial spaces. The seminiferous tubules were atrophic and lined by Sertoli cells. Occasional spermatogenic cells could be found in some cases. The leukemic cells were also observed to invade and accumulate beneath the Sertoli cell layer. Destruction of the tubules by the infiltrate was seen in advanced cases. Postradiation biopsies were characterized by fibrosis. The sigeveloped subsequent marrow relapse despite the effective control of the testicular relapse by irradiation. Another important observation is the high incidence of testicular relapse (16.2% so far).