One hundred ninety-three patients with stage II, III, or IV follicular large-cell, diffuse large-cell, diffuse mixed, immunoblastic, or diffuse small noncleaved-cell (non-Burkitt's) lymphoma were randomized to receive either cyclophosphamide 650 mg/m2 intravenously (IV), doxorubicin 25 mg/m2 IV, etoposide 120 mg/m2 IV on day 1, mechlorethamine 6 mg/m2 IV, vincristine 1.4 mg/m2 (no cap at 2 mg total dose) IV on day 8, prednisone 60 mg/m2 orally daily days 1 through 14, procarbazine 100 mg/m2 orally daily days 8 through 14, and methotrexate 500 mg/m2 IV on day 15 with leucovorin 50 mg/m2 orally every 6 hours for four doses beginning 24 hours after methotrexate with cycles repeated every 28 days (ProMACE-MOPP) or same day-1 treatment as ProMACE-MOPP plus cytarabine 300 mg/m2 IV, bleomycin 5 U/m2 IV, vincristine 1.4 mg/m2 (no cap at 2 mg total dose) IV, and methotrexate 120 mg/m2 IV on day 8, leucovorin 25 mg/m2 orally every 6 hours for four doses beginning 24 hours after methotrexate, and prednisone 60 mg/m2 orally daily days 1 through 14 with cycles repeated every 21 days (ProMACE-CytaBOM). Co-trimoxazole two double-strength tablets orally twice daily throughout the period of treatment was added to the ProMACE-CytaBOM regimen when an increased risk of Pneumocystis carinii pneumonia was found in the first 35 patients receiving this combination. Median follow-up is 5 years. Among the 99 patients treated with ProMACE-MOPP, 73 achieved a complete remission (CR) (74%), 30 complete responders have relapsed (41%), and 45 patients have died (45%), including two (2%) of treatment-related causes. Among the 94 patients treated with ProMACE-CytaBOM, 81 achieved a CR (86%), 22 complete responders have relapsed (27%), and 31 patients have died (33%). The complete response rate (P2 = .048) and survival (P2 = .046) were significantly higher for patients treated with ProMACE-CytaBOM. The mortality of ProMACE-CytaBOM treatment overall was six of 94 patients (6.4%). There was no treatment-related mortality among patients treated with prophylactic co-trimoxazole (n = 59). ProMACE-CytaBOM combination chemotherapy with co-trimoxazole prophylaxis is a safe and effective treatment for patients with aggressive histology malignant lymphoma and is superior to ProMACE-MOPP.