The incidence of primary mediastinal lymphoma in adults was investigated in 184 patients with non-Hodgkin's lymphoma. This entity was defined as disease within the mediastinum in patients who presented with symptoms due to an enlarging mediastinal mass. Of 184 patients, 17 presented with primary mediastinal lymphoma. All had a diffuse histologic pattern. The most common pathologic type was poorly differentiated lymphocytic lymphoma, diffuse (PDL-D), (11 cases). In nine of these 11 cases the patients had tumors of convoluted lymphocytes. The presentation was rapid in onset, with heart failure, pericarditis, dyspnea and superior vena caval syndrome predominating. Eleven of the 17 were clinical stage I or II, but eight of these had widespread disease on pathologic staging or rapid dissemination soon after diagnosis. In conclusion (1) primary mediastinal lymphoma is always diffuse in histology. (2) The most frequent pathologic type is PDL-D, with convoluted morphology. (3) Compression of vital intra-thoracic structures is common. (4) Although seemingly localized at presentation, this entity usually implies disseminated disease.