During a 12-month period, 31 patients underwent diagnostic and therapeutic thoracoscopy for previously undiagnosed thoracic diseases. In all patient, the diagnosis had been unobtainable by the usual diagnostic modalities of thoracentesis, closed pleural biopsy, bronchoscopy, or mediastinoscopy. The patients ranged from 4 years to 84 years old. One procedure was performed for mediastinal mass, 8 for parenchymal lesions, 21 for pleural diseases, and 1 for diaphragmatic disease. A correct diagnosis was obtained by thoracoscopy in 31 procedures for a 90% overall accuracy rate. There was no clinically significant morbidity in this series and no procedure-related mortality. Thoracoscopy, performed under intercostal nerve block and regional anesthesia, has proved to be a very attractive method of the diagnosis of thoracic disease.