Our previous study suggested a close relation between a preceding chronic tuberculous pyothorax and the development of non-Hodgkin's lymphoma (NHL) in the pleural cavity. To confirm this further, 37 cases were collected from Japanese hospitals, and their clinical and pathological findings summarized. The age at first admission for lymphoma of patients ranged from 46 to 81 (mean 63) years, the male to female ratio being 5.2:1. All patients were admitted after a 22-55 (mean 33) year history of pyothorax resulting from artificial pneumothorax for the treatment of pulmonary tuberculosis (29 cases) or tuberculous pleuritis (seven cases). The most common presenting symptom was chest pain. The main tumor mass, detected by chest roentgenogram and computed tomographic scans, was situated in the pleura (28 patients), the lung near the pleura (five patients) and the pleura and lung (four patients). The diagnosis of pleural NHL was made by biopsy for 31 of the patients and at autopsy for the other six. Histologically 30 (81%) of the 37 cases were of the diffuse large cell type, and of these the immunoblastic type was the most common (22 cases). Immunological and immunohistologic studies revealed a B-cell nature of the proliferating cells in all but one tumor. Thirty-two patients received chemotherapy and/or radiotherapy. Twenty-seven patients died between one and 144 (median eight) months of diagnosis. Autopsies carried out in 23 cases revealed the disease to have been localized to the thorax in 11 patients. These findings indicated that malignant B-cell lymphoma arose as a monoclonal growth from a pool of proliferating polyclonal B lymphocytes in tissues affected by the chronic tuberculous pyothorax.