In a consecutive series of 1,000 patients admitted since 1970 for pleural effusions, 215 with undiagnosed chronic effusions (with previous negative cytologic and needle biopsy results) underwent thoracoscopy. The investigation was usually performed under general anesthesia, originally with a 9-mm diameter cold light laparoscope, but, since 1978, with a 7-mm diameter thoracoscope of our design with biopsy forceps connected to a diathermocoagulating device. Thoracoscopy diagnosed 131 of 150 malignant effusions in the series. We observed no false positive results. A repeat pleural cytology and needle biopsy performed the day before thoracoscopy yielded only 41% positive results. The higher yield by our new thoracoscope (97% positive results, versus 78% with the laparoscope) can be accounted for by a better visualization of the pleural space, easier handling of biopsy material, and the systematic use of diathermocoagulation. Complications were rare, minor, and not life-threatening.