The percent of marrow blasts on day 7 of therapy was determined for 128 children with previously untreated acute lymphoblastic leukemia and white blood count (WBC) greater than or equal to 50,000/microliters and/or lymphomatous features enrolled in the Childrens Cancer Study Group trial of the Berlin Frankfurt Munster 76/79 regimen (CCG-193P). Patients received four-drug induction therapy including vincristine, prednisone, l-asparaginase, and daunomycin. Ninety-seven patients had fewer than 25% marrow blasts on day 7. Of these, 94 survived and maintained remission through day 28 and were designated early responders. Thirty-one patients had greater than 25% marrow blasts on day 7. Of these, 28 survived and achieved remission on day 28 and were designated late responders. The outcome of patients who underwent a day 7 marrow aspiration was similar to those who did not. Early responders had a 77.4% +/- 4.5% (standard deviation) 3-year estimated disease free survival, while late responders had 47.3% +/- 9.8% (P less than 0.001). Early responders had a superior outcome both in the subset with an initial WBC less than 50,000/microliters (P = 0.025) and in the subset with a WBC greater than or equal to 50,000/microliters (P = 0.01). The day 7 marrow response had prognostic value in this population of children with unfavorable presenting features who received four-drug remission induction therapy.