Transthoracic needle biopsy has become a frequently used method for obtaining tissue for diagnosis of pulmonary lesions. The procedure carries an inherent risk of pneumothorax and hemorrhage, the latter usually manifested by transient hemoptysis. Data on 430 patients who underwent transthoracic needle aspirations from 1968 through 1977 were studied to determine accuracy and complications of this procedure as related to lesion type and location. Malilgnant disease, most often metastatic, was present in all but 10 patients. Also, 86% of patients were more than 50 years of age and most were not candidates for operation. Sufficient tissue for diagnosis was obtained in 82% of cases. The diagnostic yield diminished significantly in central lesions less than 2 cm in size. Pneumothorax and hemoptysis or pulmonary hemorrhage were the most common complications. Biopsy of central lesions, especially those in the mediastinum, were more often associated with pneumothorax, whereas lesions near the hilar region were more susceptible to hemorrhagic complications. Two deaths occurred, both from hemorrhage after biopsy in cavitating lesions with air/fluid levels. The incidence of pneumothorax was also higher in cavitating lesions. Fewer complications occurred with biopsy of lesions along the pleural surface and lesions in the periphery of the lung.