High-dose methotrexate (500 to 33,600 mg per square meter of body-surface area) with leucovorin rescue is a common component of therapy for acute lymphocytic leukemia. To increase understanding of the relation between the serum concentration and the effect of methotrexate, we conducted a randomized, prospective study of 108 children with “standard-risk” acute lymphocytic leukemia who were treated with 15 doses of methotrexate (1000 mg per square meter) that were infused over 24 hours. The median length of follow-up was 3.5 years from diagnosis for patients still in remission. Variability between patients in methotrexate clearance produced steady-state serum concentrations that ranged from 9.3 to 25.4 μM. Patients with median methotrexate concentrations of less than 16 μM (n = 59) had a lower probability of remaining in remission (P<0.05) than patients with concentrations of 16 μM or more (n = 49). Multivariate analyses indicated that patients with methotrexate concentrations of less than 16 μM were 3 times more likely to have any kind of relapse during therapy (P = 0.01) and 7 times more likely to have a hematologic relapse during therapy (P = 0.001). Stepwise Cox's regression identified leukemic-cell DNA content, methotrexate concentration, and hemoglobin as significant prognostic variables for hematologic relapse (P = 0.0005). We conclude that there is a concentration–effect relation for high-dose methotrexate in acute lymphocytic leukemia and that 1000 mg per square meter infused over a period of 24 hours may not be optimal for patients with relatively fast drug clearance. (N Engl J Med 1986; 314:471–7.), VARIOUS studies have shown that the intensity of cancer chemotherapy, as measured by either drug dosage or dose rate, can significantly affect the clinical outcome in patients with some drug-sensitive cancers, including breast cancer,1,2 Hodgkin's and non-Hodgkin's lymphoma,3 4 5 small-cell carcinoma of the lung,6 and childhood acute lymphocytic leukemia.7 In those investigations, patients who were given full-dose chemotherapy or a larger percentage of the chemotherapy planned for them responded better than those given half doses or a lower percentage of planned chemotherapy. For most anticancer drugs, the maximum dosage is determined by host tolerance to adverse effects. One exception is methotrexate.