Thirty-four children with acute lymphoblastic leukemia, who developed bone marrow relapse after treatment was electively stopped, received reinduction, consolidation, continuing therapy, and intrathecal (IT) methotrexate (MTX). Sixteen children who relapsed within six months of stopping treatment had a median second-remission duration of 26 weeks; all next relapses occurred in the bone marrow. In 18 children who relapsed later, the median duration of second remission was in excess of two years, but after a minimum of four years' follow-up, 16 patients have so far relapsed again (six in the CNS). CNS relapse occurred as a next event in four of 17 children who received five IT MTX injections only and in two of 14 children who received additional regular IT MTX. Although children with late marrow relapses may achieve long second remissions, their long-term out-look is poor, and regular IT MTX does not afford adequate CNS prophylaxis. It remains to be seen whether more intensive chemotherapy, including high-dose chemoradiotherapy and bone marrow transplantation, will improve the prognosis in this group of patients.