Plasma methotrexate (MTX) concentrations at 48 hours were determined for 40 patients with osteosarcoma who received 256 infusions of high-dose methotrexate-citrovorum rescue (HD-MTX-CF) regimen. Five manifestations of toxicity (dermatitis, stomatitis, myelosuppression, liver dysfunction, and kidney dysfunction) were considered in the assessment of the overall toxicity. Logistic regression analysis was applied to study the effect of number of prior infusions, age, and 48-hour MTX plasma level on the risk of moderate or severe overall toxicity. Each factor had a significant effect with P less than 0.08. The predicted incidence of moderate-severe overall toxicity in the high-risk group (48-hour MTX level greater than 1.00 x 10(-6) mol/l., prior infusions greater than 10, age greater than or equal to 15 years) was 33.2% compared to only 2.4% in the "low-risk" group (48-hour MTX level less than or equal to 1.00 x 10(-6) mol/l., prior infusions less than or equal to 10, age less than 15 years). The plasma MTX determination at 48 hours postinfusion was found to be independent of both dose infused and patient's age.