The accuracy of chest computerized tomography (CT) in detecting malignant hilar and mediastinal involvement by squamous cell carcinoma of the lung is examined. The preoperative chest CT scans of 74 patients with pathologically proven squamous cell lung carcinoma were prospectively and retrospectively reviewed. Criteria for the diagnosis of malignant hilar involvement were nonvascular mass enlarging the hilum; local alteration of hilar contour; adenopathy greater than 1 cm; thickened posterior wall of the bronchus intermedius and distal upper lobe bronchi; and bronchial displacement, compression, and obstruction. Criteria for the diagnosis of malignant mediastinal involvement were confluence of tumor with the mediastinum, altered contour of the azygoesophageal recess, thickened posterior wall of the proximal main stem bronchi, and mediastinal adenopathy greater than 1 cm. Calcified hilar and mediastinal nodes were considered benign. Our results, corrected for reader error, were 92% sensitive, 92% specific, and 96% accurate in the hilum and 95% sensitive, 77% specific, and 82% accurate in the mediastinum. These data support a significant role for chest CT in the preoperative staging of non small cell lung carcinoma.