Pediatric mature B-cell non-Hodgkin lymphomas (B-NHLs) are highly aggressive malignant tumors that are curable with chemotherapy (ChT). High-dose methotrexate (MTX) is considered indispensable for successful treatment, but this therapy frequently induces severe mucositis and infectious complications, especially in induction, which can cause treatment failure. A prospective multicenter trial of combined immunochemotherapy for advanced-stage B-NHL with rituximab and the modified NHL-BFM-90 protocol was conducted. The major differences from the original protocol were a decrease in the dose of MTX from 5000 to 1000 mg/m2/24 h in the first 2 ChT blocks and the addition of rituximab at 375 mg/m2 to each of the first 4 blocks of ChT. Eighty-three newly diagnosed patients with a median age of 8.84 years with Burkitt lymphoma/leukemia and diffuse large B-cell lymphomas stage III to IV were included. Four patients died during induction ChT due to tumor lysis syndrome and infection. Two additional patients died subsequently due to tumor resistance. Complete remission was achieved in 77 (92.8%) patients; 2 patients relapsed at 1 and 3 months, and 2 developed secondary malignancies at 1 and 6.5 years, respectively, after the completion of therapy. The overall survival probability was 82%±8% with a median follow-up of 65.2 months. Combined therapy with rituximab and intensive ChT with a reduced MTX dose of 1 g/m2 in the 2 induction courses was feasible and produced high cure rates in patients with pediatric advanced-stage mature B-NHL.