Three-hundred-sixty cases of solitary fibrous tumor of the pleura from the literature are analyzed, and eight new cases are described. Of patients reported on prior to 1972, 72% had symptoms due to the tumor at the time of diagnosis, but only 54% of patients reported on since then were symptomatic. This probably reflects earlier diagnosis as a result of increased use of chest radiographs in asymptomatic populations. Cough, chest pain, dyspnea, and/or pulmonary osteoarthropathy are each found in at least one-third of patients who have symptoms. Approximately 80% of solitary fibrous tumors of the pleura originate in the visceral and 20% in the parietal pleura. In the literature and in this experience these tumors are on the whole circumscribed. The range in size from 1-36 cm with a mean of 6 cm. Many are pedunculated on pleural-based pedicles that contain hypertrophic arteries and veins. Histologic examination of the tumor usually discloses cellular areas alternating with hyalinized and/or necrotic areas. Spindle-shaped cells typically have minimal nuclear pleomorphism and rare or absent mitoses. Numerous thin-walled vessels constitute an additional feature of large tumors. Electron microscopical examination reveals features of both fibroblasts and mesothelial cells. Solitary fibrous tumors behave in a benign fashion in 88% of cases after surgical resection. In 12% of the cases the tumor is responsible for the patient's death because of its extensive intrathoracic growth, by virtue of either late diagnosis or unresectable recurrence. No single histologic feature allows a definite prognosis. The best indicator of a good prognosis is the presence of a pedicle supporting the tumor. Also favorable in circumscription of the tumor without invasion of lung, mediastinum, or chest wall. Nuclear pleomorphism and a high mitotic rate are seen in larger tumors but do not necessarily indicate a poor prognosis if the tumor is circumscribed. Solitary fibrous tumors of the pleura are not associated with asbestos.