Six hundred thirty-four children with acute lymphoblastic leukemia (ALL) were randomized to receive sanctuary therapy consisting of either cranial irradiation (CRT) plus intrathecal (IT) methotrexate (MTX) or three courses of intermediate-dose methotrexate (IDM) plus intrathecal methotrexate. Two hundred sixty-six male patients achieved a complete response and were evaluable for the effects of prophylactic therapy on the duration of remission. There was one isolated testicular relapse (0.8%) in the IDM group compared with 14 (10%) in the CRT group. The incidence of testicular relapse was significantly lower in the patients treated with IDM (P less than 0.001). High plasma levels of MTX achieved during the 24-hour infusions may result in increased penetration of MTX into the interstitium of the testes, thus allowing for the eradication of sequestered leukemic cells and preventing the emergence of drug resistance resulting from exposure to sublethal concentration of MTX.