RATIONALE: Radiolabeled monoclonal antibodies, such as iodine I 131 tositumomab, can find cancer cells and carry cancer-killing substances to them without harming normal cells. Monoclonal antibodies, such as rituximab, can block cancer growth in different ways. Some block the ability of cancer cells to grow and spread. Others find cancer cells and help kill them or carry cancer-killing substances to them. Drugs used in chemotherapy, such as cyclophosphamide, doxorubicin, vincristine, and prednisone, work in different ways to stop the growth of cancer cells, either by killing the cells or by stopping them from dividing. Giving a radiolabeled monoclonal antibody together with rituximab and combination chemotherapy may kill more cancer cells. PURPOSE: This phase II trial is studying the side effects of giving iodine I 131 tositumomab together with rituximab and combination chemotherapy and to see how well it works in treating patients with previously untreated stage II, stage III, or stage IV follicular non-Hodgkin lymphoma.

OBJECTIVES: * To evaluate the response rate in patients with previously untreated stage II-IV follicular non-Hodgkin lymphoma treated with rituximab, cyclophosphamide, doxorubicin hydrochloride, vincristine sulfate, and prednisone (R-CHOP) in combination with iodine I 131 tositumomab. * To evaluate the toxicity of this regimen in these patients. * To estimate the 3-year progression-free survival rate in patients treated with this regimen. * To estimate the 5-year progression-free and overall survival rate in patients treated with this regimen. * To assess the safety profile of this regimen in these patients. OUTLINE: This is a multicenter study. * Induction therapy: Patients receive R-CHOP\* comprising rituximab IV, cyclophosphamide IV, doxorubicin hydrochloride IV, and vincristine sulfate IV on day 1 and oral prednisone once daily on days 1-5. Treatment repeats every 21 days for up to 6 courses in the absence of disease progression or unacceptable toxicity. Patients with at least stable disease then proceed to consolidation therapy. NOTE: \*Patients receive R-CHOP in courses 1-4 and CHOP alone in courses 5 and 6. * Consolidation therapy: Within 12 weeks after completion of induction therapy, patients receive tositumomab IV over 1 hour followed by a dosimetric dose of iodine I 131 tositumomab IV over 20 minutes. Patients then undergo whole body gamma camera scans over a 1-week period to determine the rate of total body clearance of radioactivity and the therapeutic dose of iodine I 131 tositumomab. Within 7-14 days after the dosimetric dose, patients receive tositumomab IV over 1 hour followed by a therapeutic dose of iodine I 131 tositumomab IV over 20 minutes. Patients with at least stable disease then proceed to maintenance therapy. * Maintenance therapy: Beginning approximately 1 year after study entry and no more than 28 days after restaging, patients receive rituximab IV every 3 months for up to 4 years (16 courses) in the absence of disease progression or unacceptable toxicity. After completion of maintenance therapy, patients are followed annually for up to 7 years. Patients who do not complete maintenance therapy are followed every 6 months for 2 years and then annually for up to 7 years.