In a retrospective review which covered the whole Dutch childhood population of approximately 3 million children we studied the prognosis in 164 children with acute lymphoblastic leukemia (ALL) who were initially treated between 1973 and 1983, and who had an isolated bone marrow relapse occurring as first relapse. Until their first relapse, the patients were initially treated according to standard protocols, while treatment for relapse was heterogeneous, and not intensive. Second complete remission (CR) was attained by 78% of the patients. The median duration of second CR was 9 months, the median survival 13 months. Multivariate analysis showed that the duration of the first CR was the most significant variable with regard to prognosis. None of the patients who developed their bone marrow relapse during initial treatment, i.e., within 24 months from diagnosis, survived. Among the 73 patients who relapsed after cessation of the initial treatment there were 19 long-term disease-free survivors, 14 of whom had not developed subsequent relapses after 48(+)-125 + months. From this study we conclude that treatment in children with first bone marrow relapse has to be intensified.