Seven children receiving maintenance therapy for acute lymphoblastic leukemia (in remission) were given oral lithium carbonate for 12 weeks. Hematologic values, serum immunoglobulin levels, and responsiveness of peripheral blood mononuclear cells to mitogen stimulation were studied during the lithium treatment period, and compared to measurements made during a 12-week period when no lithium was administered (children served as their own controls). Changes attributable to treatment with lithium carbonate included increases in total white blood cell count, circulating neutrophil count, and response to phytohemagglutinin-P. No important toxicity to lithium carbonate was observed.