One hundred two adult patients with stage III1A (76 patients) and stage III1B (26 patients) Hodgkin's disease were treated with two cycles of mechlorethamine, vincristine, procarbazine, and prednisone (MOPP) and radiotherapy (XRT) between 1970 and 1984. Sixty-four of the patients were treated between 1970 and 1978 with two cycles of MOPP and XRT to the mantle, upper abdomen, and pelvis. The remaining 38 patients were treated from 1978 to 1984 with a modification of the protocol in which pelvic XRT was omitted and low-dose whole-lung XRT was administered to patients with unfavorable mediastinal disease. The 10-year actuarial freedom-from-progression (FFP) and determinate survival rates at a mean follow-up of 93 months were 84% and 86% for stage III1 disease, 86% and 84% for stage III1A disease, and 78% and 91% for stage III1B disease. Three patients died of treatment-related toxicities without evidence of Hodgkin's disease, two died of complications of myelosuppression and one of acute nonlymphocytic leukemia (ANLL). Neither FFP nor determinate survival rates were significantly influenced by B symptoms, unfavorable mediastinal disease, histologic subtype, extent of abdominal disease, the omission of pelvic XRT, the use of whole-lung XRT, or the number of splenic nodules. Patients 40 years of age or older had a 73% determinate survival rate at 10 years compared with 88% for patients younger than 40 years (P = .01). This survival difference was due to treatment-related toxicity in the older group. This study indicates that two cycles of MOPP and XRT to the mantle and upper abdomen is as effective as more intensive treatment for all patients with stage III1 Hodgkin's disease. This treatment program can preserve fertility and has had only a 1% actuarial incidence of ANLL at 15 years.