Twenty patients (17 women, three men) with mediastinal diffuse large-cell lymphoma with sclerosis are reported. At the time of diagnosis, the disease was confined to supradiaphragmatic areas in all patients but two, who had kidney involvement (seven were stage I, 11 were stage II, and two were stage IV). A B-cell phenotype was demonstrated in nine of the 11 cases that were analyzed for cell lineage. All patients received an anthracyclin-containing regimen followed by mantle radiotherapy. Analysis of pretreatment characteristics showed that only the extent of disease influences outcome. Moreover, achievement of complete remission (CR) after chemotherapy appears to be a major prognostic factor. Two-year survival was better in patients who reached CR after chemotherapy than in those who did not (100% versus 9%; p less than 0.0001). Overall 2-year and 7-year survival rates were 50% and 33%, respectively. Therefore, localized mediastinal large cell lymphoma with sclerosis should be considered a high-risk subtype of non-Hodgkin's lymphoma in which standard treatment approaches are unsuccessful.