The differential diagnosis of pleural effusion is large but has not included asbestos exposure. For 1 to 9 years we followed 12 patients with "idiopathic" effusions that were frequently recurrent, usually bilateral, and often followed by continued chest pain. The fluid was a sterile, serous, or blood-tinged exudate. Physical findings were limited to clubbing, dry rales, and signs of effusion. Asbestos exposure was from 3 to 38 years, but often this history was elicited only with difficulty. Usually, mechanics of breathing were normal, lung diffusing capacity reduced, and alveolar-arterial Po2 differences elevated. Decortication and lung biopsy specimens in seven patients and postmortem examination in one showed nonspecific pleuritis with rare asbestos bodies and fibers; all had various degrees of chronic interstitial pneumonitis with asbestos bodies. One case of mesothelioma was recognized 9 years after the first documented effusion. With better understanding of the dangers of asbestos and better surveillance of workers, asbestos pleural effusion will be recognized oftener. It occurred in 21% of all patients with asbestosis seen at our laboratory.