Of 107 patients with diffuse histiocytic lymphoma (DHL) seen at the University of Chicago, 14 (13%) were classified as having moderate to marked sclerosis. Three of the 14 (21%) had predominantly retroperitoneal masses. Fifty percent of our group, however, had bulky disease seen predominantly or exclusively in the mediastinum, and all of these individuals had superior venacaval (SVC) obstruction. Of the seven patients with SVC syndrome, three were in Pathologic Stage IIA, three were in Clinical Stage II, and only one was in Clinical Stage IIIA. No other patients with DHL displayed SVC obstruction or predominantly mediastinal disease. Five of seven patients with SVC syndrome had large cleaved cell histology. In spite of an apparently favorable histopathologic subtype and a tendency to localized involvement, patients with DHL and sclerosis who have bulky or disseminated disease appear to be resistant to megavoltage radiotherapy alone and relatively resistant to combination chemotherapy.