To determine useful prognostic factors in treating childhood acute lymphoblastic leukemia (ALL), we correlated conventional risk factors and bone marrow response 14 days after induction chemotherapy. Our study included 116 precursor B-cell (n = 104) and T-cell (n = 12) ALL patients treated with our protocol between 1988 and 1999. The patients were classified into 3 initial risk groups on the basis of conventional risk factors (56 in the low-risk, 33 in the high-risk, and 27 in the very high-risk groups). All patients received similar systemic chemotherapy regimens before the evaluation of their bone marrow on day 14. We evaluated the marrow of 69 patients as M1 (less than 5% blasts), 25 as M2 (5%-25% blasts), and 22 as M3 (more than 25% blasts). Although all patients attained an initial complete remission (CR), relapse was noted in 33 of the 116 patients, and 15 patients died. All of the M1 marrow patients, irrespective of the initial risk group, showed the best event-free survival rate (85.1% +/- 3 4.4%), the lowest relapse rate (14.5%), and the highest attainment of a second CR (100%); they were defined as the new R1 prognostic group. The low-risk patients with M2 or M3 marrow (R2 group) had a relatively high relapse rate, but all of these relapsed patients were treated successfully with subsequent therapy. High- or very high-risk patients with M2 or M3 marrow (R3 group) had the worst prognosis. Our new prognostic definition (R1, R2, R3) incorporating day 14 marrow findings is useful to tailor early-phase treatments for better therapeutic results in childhood ALL.