Analysis of the results of radiotherapy in patients with non-Hodgkin's lymphomas revealed 26 who had localized diffuse histiocytic lymphoma characterized by a median survival of 13 months and a high risk of relapse within one year of treatment. In contrast, 34 patients with localized nodular mixed histiocytic-lymphocytic or nodular lymphocytic poorly differentiated lymphoma exhibited a median survival of 80 months and a pattern of continued late relapse. Patients with localized extralymphatic involvement associated with limited nodal disease behaved similarly to patients with lymphoma confined to limited nodal sites.