The evidence for the safety and efficacy of adding rituximab to intensive chemotherapy in pediatric patients with aggressive mature B cell non-Hodgkin lymphoma/leukemia (B-NHL/B-AL) is not yet robust. In this prospective multi-institutional trial, 419 evaluable patients <= 16 years of age with newly diagnosed B-NHL/B-AL were enrolled. Patients were stratified into 4 risk groups according to stage, resection status, and serum lactate dehydrogenase. Patients in group R1 received 3 therapy courses in the treatment order A-B-A. Patients in group R2 received 5 courses A-B-A-B-A. Patients in group R3 received 6 courses A-BB-AA-BB-AA-BB. For patients in group R4, rituximab was added to the chemotherapy backbone for patients in R3 (A-RBB-RAA-RBB-RAA-BB). At a median follow-up of 54 months, the 4-year event-free survival (EFS) for the entire group was 88.3 +/- 1.6% (76.0 +/- 4.3% in the historical study). The EFS rates according to the intention-to-treat principle were 100%, 98.6 +/- 1.2%, 94.2 +/- 1.8%, and 73.5 +/- 3.7% for patients in treatment groups R1, R2, R3, and R4, respectively (P < 0.001). There were 9 (2.1%) toxic deaths due to infection during treatment. Regarding the toxicities of rituximab, grade 3/4 thrombocytopenia, mucositis, and infection occurred in 44.0%, 33.3%, and 64.0% after courses R-BB and grade 3/4 neutropenia, thrombocytopenia, and infection occurred in 96.3%, 77.8%, and 54.1% after courses RAA. The addition of rituximab to intensive chemotherapy is feasible even in a developing country. EFS was significantly improved when compared with the historical data.