Thoracoscopy was carried out in 81 cases of diffuse pulmonary disease in order to obtain lung tissue for biopsy. After we established artificial pneumothorax, the thoracoscope was introduced under local anesthesia, multiple biopsy specimens (theta 3 mm) were obtained under visual control, and an underwater sealed drain was left in place. The method was used to determine the cause of x-ray shadowing and respiratory distress in 26 immunocompromised patients. Within 2-48 hours, all biopsy specimens provided sufficient microbiologic and morphologic information to guide management, eg, specific antimicrobial drugs, decreasing or intensifying immunosuppression, or cytostatic therapy. Thoracoscopy was tolerated better than fiberoptic bronchoscopy, especially in hypoxic patients. Persisting or recurring pneumothoraces were seen in four patients and was not a major complication. In one very ill patient, the spleen was punctured accidentally before biopsy specimens were taken. Of 63 nonimmunocompromised patients, a histologic diagnosis was obtained in 57 (90 percent). In most of these patients, previous biopsy procedures had produced inconclusive results. Also in this group persisting or recurring pneumothoraces were seen in four patients, but closed eventually in a conservative way.