To explore the clinical significance of the concentration of methotrexate (MTX) in erythrocytes (E-MTX), 42 boys and 31 girls were studied during maintenance chemotherapy for childhood acute lymphoblastic leukemia for periods of 3-22 months (median, 8 months) at an unchanged dose of MTX. For each study period, a weighted mean of white cell counts (mWBC), absolute neutrophil counts (mANC), and serum aminotransferases (mAT) were calculated, using as weights the intervals from sampling until the next WBC, ANC, or AT determinations were done. In 17 patients who underwent at least six measurements of E-MTX during a period in which the MTX dose remained unchanged for up to 22-months, the median intraindividual coefficient of variation for E-MTX was 10% (range, 5%-22%). For each patient, a mean of all E-MTX values (mE-MTX) during a study period (range, 1-15 measurements; median, 3) was used as an index of the RBC accumulation of MTX at the prescribed dose of MTX. Among 42 patients receiving full-dose MTX (greater than 17.5 mg/m2), the mE-MTX ranged between 3.4 and 9.6 nmol hemoglobin (Hb) (interindividual coefficient of variation, 33%). The mE-MTX was significantly related to the MTX dose (r = 0.45, P = 0.00003). The mWBC and mANC were both significantly related to the mE-MTX (mWBC: r = -0.31, P = 0.004; mANC: r = -0.35, P = 0.02), but not to the dose of MTX (mWBC: r = -0.08, P = 0.25; mANC: r = -0.22, P = 0.08). Each of four patients with a persistent rise in AT above the upper normal limit (40 IU/l) and an mAT of greater than 80 IU/l had an mE-MTX of greater than 6.5 nmol/mmol Hb. Due to its low intraindividual variation, E-MTX may be useful for detecting persistent or intermittent failure of patient compliance. Its prognostic significance and its clinical value in MTX dose adjustment should be explored in prospective studies.