Thirty children with acute lymphoblastic leukemia were monitored with serial serum ferritin determinations for up to 17 months. In children with acute lymphoblastic leukemia before initiation of therapy, or in relapse, the mean serum ferritin concentration was 636 microgram/l. In children who went into primary remission. the mean serum ferritin concentration fell from 265 microgram/l prior to start of treatment, to 161 microgram/l after 3 months of treatment. Five patients relapsed. Their serum ferritin levels prior to the relapses ranged from 7 to 135 microgram/l. At the time of relapse a further increase in serum ferritin was found in only 2 of the children. Thus, whereas high serum ferritin levels may signal disease activity in acute lymphoblastic leukemia, a normal serum ferritin level does not exlude disease activity or impending relapse.