As the intensity of cancer chemotherapy has been reported to influence clinical response for several drug-sensitive cancers, we have investigated the relation between systemic exposure to high-dose methotrexate (HDMTX) and clinical response in childhood acute lymphocytic leukemia (ALL). A total of 108 consecutive, previously untreated children with "standard-intermediate risk" ALL were randomized to receive postinduction therapy with HDMTX (1000 mg/m2 iv over 24 hours weekly for 3 weeks, then every 6 weeks for 72 weeks), superimposed on conventional therapy with low-dose 6-mercaptopurine (6MP; 50 mg/m2 orally per day) and methotrexate (MTX; 25 mg/m2 orally per week). The systemic clearance of HDMTX ranged from 40 to 131 ml/minute/m2 among these patients, yielding MTX steady-state plasma concentrations (Cpss) ranging from 9.3 to 25.4 microM during the infusion. The group of patients (n = 59) with median MTX Cpss less than 16 microM during the HDMTX infusion had a higher probability of having any relapse than patients (n = 49) with MTX Cpss greater than 16 microM (P less than 0.05). In a previously reported univariate analysis, patients with MTX Cpss less than or equal to 16 microM were 3.2 times more likely to relapse on therapy (P = 0.01) and 6.9 times more likely to have a hematologic relapse on therapy (P = 0.001). Multivariate and stepwise Cox's regression analyses indicated that MTX Cpss retains its prognostic importance even when other prognostic variables (i.e., DNA Index, WBC, hemoglobin) are considered.(ABSTRACT TRUNCATED AT 250 WORDS)