Transbronchial lung biopsy (TBB) was performed during fiberoptic bronchoscopy under fluoroscopic guidance in 107 patients. TBB was diagnostic in 17 of 21 (81 percent) cases with localized malignant lesions greater than 4 cm in diameter and bronchial brushing was positive in 12 (57 percent). TBB was diagnostic in 14 of 24 (58 percent) cases with localized malignant lesions equal to or less than 4 cm in diameter, whereas brush biopsy was positive in 7 (29 percent). In 13 cases with suspected Pneumocystis carinii pneumonia, the combination of TBB and brush biopsy was diagnostic of P carinii pneumonia in all 11 patients; TBB was diagnostic in 10 and brush biopsy diagnostic in 5. Cytomegalovirus pneumonitis was diagnosed in the other two cases. In 20 patients with localized infiltrates or nodules, a TBB diagnosis of acute or chronic inflammation excluded malignancy in 15 of 18 cases (follow-up 3 to 24 months) and tuberculosis was diagnosed in 2. Malignancy was found in one patient with acute inflammation on TBB. TBB accurately diagnosed 23 of 29 (79 percent) cases of diffuse lung disease. Following TBB, one patient had pneumothorax and nine patients had hemoptysis not requiring treatment. TBB and bronchial brushing via the flexible fiberoptic bronchoscope offer good diagnostic accuracy and a low complication rate in both diffuse and localized lung diseases.