From January 1970 to March 1981, localized diffuse histiocytic lymphoma (DHL) was identified in 31 patients by exploratory laparotomy and splenectomy (pathologic stage I, 17 patients; pathologic stage II, 14 patients) at the University of Chicago. The median follow-up time was 72 months. All patients were previously untreated and received radiation therapy as their primary treatment modality. Chemotherapy was administered only at the time of relapse. All but two patients achieved a complete remission (CR) with radiation therapy. The actuarial disease-free survival for patients with stage I disease is 94% at 5 years and 72% at 10 years. For stage II disease, the disease-free survival is 56% at 5 years and 31% at 10 years. The difference in the disease-free survival between stage I and II is statistically significant (P = .02). The survival at 10 years is 70% for stage I disease and 46% for stage II disease. Five patients had documented relapses (four had stage II disease). Only two of those who relapsed achieved a second CR with salvage chemotherapy. Our data show an excellent outcome in patients with pathologic stage I disease, indicating that a high percentage of these cases can be cured with radiotherapy alone. Patients with clinical stage II disease might be served better with chemotherapy.