Findings from the first 430 CT examinations of the thorax performed at the Mayo Clinic were compared with results of film studies and the most certain later diagnoses in the same cases. Results were analyzed by disease and by mediastinal compartment. CT was superior in staging bronchogenic carcinoma, detecting abnormal masses, and in demonstrating mediastinal normally when plain films were equivocal. It generally gave clear delineations of lesions and distinguished them better from normal structures, and, in correlation with clinical data, it was more accurate in suggesting whether they were benign or malignant.