OBJECTIVES: I. Identify response related factors predictive of relapse among children with previously untreated standard risk acute lymphoblastic leukemia (ALL). II. Determine the prognostic significance of residual leukemic blasts at specific times during induction therapy: M3 marrow status (greater than 25% blasts) at day 7; M2 status (5%-25% blasts) at day 14; and residual circulating leukemic blasts at days 7 and 14. III. Determine the prognostic significance of residual leukemic burden, as measured by fluorescence activated cell sorting/leukemic progenitor cell assay, on marrow aspirates acquired at the end of induction therapy, at the beginning of maintenance therapy, and at the completion of all therapy in patients with B precursor ALL. IV. Determine the prognostic significance of residual t(1;19) detected in marrow aspirates by PCR-based analyses of the fusion transcript E2A-PBX1 at the end of induction therapy, at the beginning of maintenance therapy, and at the completion of all therapy. V. Examine the interrelationships among these response-related prognostic factors and their correlation with ploidy, karyotype, and immunophenotype. VI. Determine, in a randomized study, whether substitution of oral thioguanine (TG) for oral mercaptopurine (MP) during consolidation, interim maintenance, and maintenance therapy improves event-free survival for patients with standard-risk ALL. VII. Study and compare the cellular pharmacokinetics of oral MP and oral TG during interim maintenance and maintenance therapy in selected patients. VIII. Compare the concentrations of MP and TG red blood cell metabolites (i.e., nucleotides, nucleosides, free bases, and methylated metabolites) during interim maintenance and maintenance therapy, and determine whether low levels of metabolites predict relapse in selected patients. IX. Determine the activities of thiopurine methyltransferase and hypoxanthine guanine phosphoribosyl transferase several times during interim maintenance and maintenance treatment, and compare the activities between the two thiopurine treatment groups in selected patients. X. Compare the incidence of central nervous system (CNS) relapse and event-free survival in patients receiving intrathecal methotrexate (MTX) vs. triple intrathecal chemotherapy (MTX/cytarabine/hydrocortisone) for presymptomatic CNS treatment. XI. Determine whether cerebrospinal fluid (CSF) terminal deoxynucleotidyl transferase (TdT) positivity predicts for CNS or marrow relapse by measuring TdT activity on CSF cytospins in cases with low white blood cell count (less than 5 cells per cubic millimeter) and suspected or questionable "blasts" during maintenance therapy. XII. Determine event-free survival in patients with standard-risk ALL and M3 marrow at day 14 when treated with intensive therapy designed for higher-risk ALL.