Ten newly diagnosed patients, with localized diffuse histiocytic lymphoma classified according to the Lukes-Collins classification, were evaluated for response to treatment with extended-field radiation. All classified tumors were of follicular center cell type (four large cleaved, four large noncleaved, and one small noncleaved). There were nine patients with stage I disease and one with stage II disease. The patient with stage II disease had only two sites of involvement. The disease was nodal in six patients and extranodal in four. All patients are alive without evidence of disease recurrence from 19 to 102 months since diagnosis (median survival, 58 + months). No differences were found in response or survival rates between histologic categories. No significant immediate or long-term complications have occurred. These results suggest that radiotherapy alone is curative for localized diffuse histiocytic lymphoma and may be superior to recently advocated chemotherapeutic approaches.