Despite improved techniques--such as bronchoscopy and percutaneous needle biopsy--to evaluate pulmonary nodules, there are still many cases in which surgical resection is necessary before carcinoma can be differentiated from benign lesions. The present study was undertaken to determine if the presence of an air bronchogram or air bronchiologram (patent visible bronchus or bronchiole) is useful in distinguishing small lung cancers from benign nodules. Thin-section chest CT scans were obtained in patients with 20 peripheral lung cancers less than 2 cm in diameter (18 adenocarcinomas, one squamous cell carcinoma, and one large cell carcinoma) and 20 small benign nodules (eight hamartomas, seven tuberculomas, two foci of aspergillosis, one focus of cryptococcosis, one chronic focal interstitial pneumonitis, and one plasma cell granuloma). The images were compared with regard to the patency of any bronchus or bronchiole within the lesions. After surgical resection, the specimens were inflated with agar and sectioned transversely to correlate gross morphology and low-power histologic sections with the CT appearance. An air bronchogram or air bronchiologram was seen in the tumors on 65% of CT scans and 70% of histologic sections. Benign nodules had a patent bronchus or bronchiole on CT scans and histologic sections in only one case (5%). These findings suggest that the presence of an air bronchogram in a lung nodule is a useful finding to help differentiate adenocarcinomas from benign lesions.