Between 1985 and 1989, 27 patients underwent thoracoscopy for an undiagnosed pleural effusion. Prior thoracocentesis (in all patients) and a needle pleural biopsy (in 15 patients) had been nondiagnostic. The rigid thoracoscope was used in 21 patients, the cystoscope in 4 patients, and the flexible bronchoscope in 2 patients. We had one mortality, and there was no operative morbidity. Thoracoscopy was diagnostic in 24 of the 27 patients (4 benign, 20 malignant). The average duration of stay in the hospital was 48 hr. We conclude that thoracoscopy is an important diagnostic aid in the management of a pleural effusion of ambiguous origin and that it is a relatively safe procedure that can be performed with an acceptable morbidity and mortality.