We evaluated the prognostic significance of mediastinal involvement in Hodgkin's disease in 79 consecutive newly diagnosed patients treated with curative-intent, nodal radiotherapy. Mediastinal masses were classified large or small depending on whether the ratio of the largest transverse diameter of the mass to the transverse diameter of the thorax at T5-6 was greater than or equal to 35. Forty-eight patients had mediastinal disease; 20 had large masses, and 28 small masses. Complete remissions were achieved in 19 patients with large masses, 26 with small masses and all patients with no mediastinal masses. Relapse have occurred in 74% of patients with large masses but in only 27% with small masses and 19% without masses (P < .001). This high recurrence rate among patients with large masses could not be explained by other known adverse prognostic factors. Survival was adversely influenced by mediastinal mass size (P = .03). We conclude that curative-intent, nodal irradiation is inadequate therapy for patients with large mediastinal masses. Controlled studies are needed to determine if survival can be improved by the addition of chemotherapy or whole lung irradiation.