The current cure rate of 80% in childhood acute lymphoblastic leukemia (ALL) attests to the effectiveness of risk-directed therapy developed through well-designed clinical trials. The ongoing Total Therapy Study XV at St. Jude Children's Research Hospital was designed to further increase cure rate and to improve quality of life. The study consists of intensive systemic and intrathecal therapy but does not include cranial irradiation, irrespective of a patient's risk features. The intensity of postremission consolidation, continuation and reinduction therapy is based on the level of minimal residual disease at the end of induction, as measured by both flow cytometric detection of aberrant immunophenotypes and polymerase-chain-reaction amplification of clonal antigen-receptor gene rearrangements. Status of thiopurine methyltransferase is determined prospectively for treatment modification. Pharmacogenetic, pharmacodynamic, gene expression and proteomic profiling studies of host normal cells and leukemic cells are performed in parallel to elucidate the mechanisms of drug resistance and to advance our understanding of leukemogenesis.