Conversions of leukemic cell lineage (lymphoid or myeloid) have been reported only rarely. Our review of the cytochemical and immunophenotypic features of 89 cases of childhood leukemia in marrow relapse indicated lineage switch (lymphoid to myeloid or the reverse) in six patients (6.7%). Five patients with acute lymphoblastic leukemia (ALL) at diagnosis had converted to acute nonlymphoblastic leukemia (ANLL), and one had converted from ANLL to ALL. Each child received lineage-specific multiagent chemotherapy when initially diagnosed, and all achieved a complete remission. After conversion, four patients readily achieved second remissions with treatment for the phenotype evident at lineage switch. Two patients with ANLL at conversion failed ALL-directed reinduction, while one of the two responded to high-dose cytarabine but died during bone marrow hypoplasia, emphasizing the importance of prompt recognition of lineage switch and selection of an appropriate plan of retreatment. Cytogenetic studies disclosed evidence of clonal selection in one patient and clonal stability in two. These findings indicate an unexpectedly high frequency of lineage switch in patients who relapse in the bone marrow after intensive chemotherapy. Although specific causative factors could not be identified, our observations suggest at least two general mechanisms for lineage switch in acute leukemia. In one, chemotherapy appears to eradicate the dominant clone present at diagnosis, permitting expansion of a secondary clone with a different phenotype. In the second, drug-induced changes in the original clone may either amplify or suppress differentiation programs so that phenotypic shift is possible.