Transbronchial lung biopsy was performed via the fibreoptic bronchoscope in 433 patients. In 183 patients, whose chest radiographs showed bilateral diffuse shadows a histologic diagnosis was established by transbronchial biopsy in 61%, including sarcoidosis (58), fibrosing alveolitis (18) and carcinoma (14). Histologically normal lung or inadequate pulmonary tissue for diagnosis was obtained from 71 patients in this group. In 225 patients with solitary chest radiography opacity, a diagnosis of bronchial carcinoma was finally made in 130, of whom 82 (63%) had a histological diagnosis established by transbronchial biopsy. In 84 patients finally shown to have sarcoidosis, 65 (77%) had positive histology by transbronchial biopsy whereas only 19 (40%) of 48 patients with fibrosing alveolitis yielded histology compatible with the diagnosis. Pneumothorax developed in three patients after the procedure but only one required intercostal drainage. Haemorrhage of 50-100 ml occurred in seven patients and bleeding ceased spontaneously in all cases. There were no fatalities.