Forty-one children with refractory acute nonlymphocytic leukemia (ANLL) were treated from March 1975 to February 1979 with a schedule-dependent combination of methotrexate (MTX) and L-asparaginase. Intravenous (IV) MTX was followed 24 hours later by IV L-asparaginase (10,000 units [U]/m2). The MTX dose was started at 60 to 100 mg/m2 and was escalated by 20 to 40 mg/m2 as tolerated. This sequence was repeated every 7 to 10 days. Eight patients (20%) achieved a complete remission (CR) and six others had a partial response (PR), with clearance of blasts from the peripheral blood and reduction of bone marrow blasts to less than 25% of nucleated marrow cells. Responding patients received a median maximum MTX dose of 120 mg/m2 (range, 60 to 220 mg/m2). The median number of courses required to achieve a CR was 6 (range, 2 to 13 courses). Toxicity consisted of allergic reactions to L-asparaginase (n = 12), stomatitis (n = 6), minimal elevation of hepatic enzymes (n = 2), and hyperglycemia (n = 1). Treatment was given on an outpatient basis in 95% of all courses. The data indicate that this combination therapy has antileukemic activity and is relatively nontoxic in childhood ANLL.