CT-guided fine-needle aspiration biopsy (FNAB) was performed on the patients with pulmonary or mediastinal masses to obtain material for cytologic/histologic diagnosis. Diagnostic accuracy and safety of the technique were evaluated in 75 patients affected with thoracic lesions still undiagnosed after thorough radiological and endoscopic investigations. The cytologic and/or microhistologic samples allowed a correct diagnosis to be made in 61 cases (81%), with no false positives and 7 false negatives (9%). The samples were inadequate for diagnostic purposes in 7 cases (9%). Specificity, sensitivity, and diagnostic accuracy were 68%, 76%, and 81%, respectively. No major complication was recorded and a case of asymptomatic pneumothorax resolved spontaneously within 48 hours. CT-guided FNAB of space-occupying lesions in the lung and mediastinum is therefore a rapid and valuable diagnostic tool and is quite safe when performed by skilled operators.