OUTLINE: This is a multicenter study. Patients are stratified according to the number of risk factors (0-2 vs 3-5). Risk factors include age (no greater than 60 vs greater than 60), tumor stage (II vs III or IV), number of extranodal sites (no more than 1 vs more than 1), performance status (0-1 vs 2-4), and serum LDH level (no greater than normal vs greater than normal). Patients receive rituximab IV on day 1; cyclophosphamide, doxorubicin, and vincristine IV on day 3; and oral prednisone on days 3-7. Patients over 60 also receive filgrastim (G-CSF) subcutaneously beginning on day 4 and continuing until blood counts recover (all other patients receive G-CSF as secondary prophylaxis). Treatment continues every 3 weeks for 4 courses in the absence of disease progression or unacceptable toxicity. Patients who respond receive 2 more courses. Patients who have no measurable disease after 6 courses receive rituximab IV once weekly for 4 consecutive weeks. This treatment continues every 6 months for 4 courses in the absence of disease progression or unacceptable toxicity. Patients who have measurable disease after 6 courses of chemotherapy receive 2 more courses for a maximum of 8 courses of CHOP, followed by maintenance therapy with rituximab (as described above). Patients are followed every 6 months for 2 years.