The case-reports of 30 patients (16 men and 14 women) with non-Hodgkin lymphoma revealed by a mediastinal mass are reviewed. The tumour was apparently localized (stages I and II) in 63% of the cases, and was associated with pleural effusion in 50%. Two histological types were identified: diffuse lymphoblastic lymphoma and diffuse large cell lymphoma, including 10 cases where the large cells had irregular nucleus and clear cytoplasma and were accompanied by fibrosis. Lymphoblastic lymphomas differed from large cell lymphomas in that they were more common in men and had greater tendency to dissemination, particularly in the meninges and bone marrow. The median survival time was 15 months, but a longer survival could be obtained in patients in complete remission. There was no difference in prognosis between localized and disseminated lymphomas. Radiotherapy of the mediastinum had no effect on survival.