Between February 1997 and March 1999, 79 consecutive CT-guided automated needle biopsy procedures were performed in 78 patients (79 small pulmonary nodules measuring 20 mm or less in diameter). In 69 (87%) of 79 lesions, sufficient material for histologic diagnosis was obtained. We evaluated the factors affecting diagnostic accuracy in CT-guided automated needle biopsy. Lesion diameter, depth from skin, chronic pulmonary disease and location were analyzed. The diameter of the lesion was a significant factor contributing to diagnostic accuracy (p = 0.0108). Pneumothorax occurred in 18 cases (23%). In 5 cases (6%), chest tube placement was necessary. Postbiopsy hemoptysis or hemosputum occurred in 23 cases (29%). In conclusion, lesion diameter was a critical factor in diagnostic accuracy. CT-guided automated needle biopsy is a feasible, useful and safe technique for the histological diagnosis of small lung lesions, but especially those larger than 8 mm in diameter.