Childhood nonlymphocytic leukemia comprises a minority (25%) of pediatric leukemia cases, yet contributes a significant proportion of overall leukemia mortality. Improvements in supportive care (antibiotics, antifungals, nutrition, and blood products) along with aggressive induction therapies have significantly improved remission induction rates over the past two decades. Ideal treatment to completely eliminate residual disease following remission is not yet known. In most series, only one out of three patients are long-term survivors of this disease. Recent advances in allogeneic bone marrow transplantation and improved techniques for autologous engraftment leave promise for significant improvements in postinduction disease control. Biologic studies of surface immunophenotype have contributed to our understanding of the heterogeneity of this family of disorders and allowed identification and characterization of leukemias of mixed myeloid/lymphoid lineage. Karyotype studies have identified important subsets of ANLL with distinctive clinical and biologic properties, for which tailored therapies someday may be developed. In addition, studies of oncogenes provide insight into regulation of leukemic hematopoiesis with potential of identifying future methods to regulate proliferation of the leukemic clone.