The therapeutic benefit of maintenance chemotherapy beyond three years for children with acute lymphocytic leukemia (ALL) in continuous complete remission was evaluated by the investigators of Childrens Cancer Study Group (CCSG). Two hundred and twenty leukemic children in first remission for three years or longer and who had received at least three years of continuous chemotherapy were eligible. One hundred and one patients were randomized to either continue chemotherapy for an additional three years or to discontinue therapy, and 119 patients nonrandomly continued or discontinued therapy. The patients had received a variety of chemotherapy regimens. The study period extended from April 1970 until December 1977, with a median follow-up time of 25 months. Relapses occurred in 15 randomized patients (15%). Randomized patients remaining on chemotherapy experienced a statistically significant lower relapse rate than patients randomized to discontinue therapy. Also among randomized patients, bone marrow relapse was significantly more frequent in males than in females. Considering the total patient group, age and white blood count at diagnosis had no significance in predicting relapse. Of relapse events in males, 21% were isolated testicular relapses, identifying the testicles as a major risk site in males completing three years of continuous complete remission. This study demonstrates that continuing chemotherapy beyond three years results in a significant prolongation of remission in males, although the eventual survival outcome for later discontinuance of therapy will require longer follow-up.