BACKGROUND: To evaluate how efficaciously and safely we can make transthoracic lung biopsy with an 18-gauge automated biopsy device. METHODS: We performed 130 transthoracic needle biopsies including 16 repeat biopsies in 114 patients with a pulmonary mass using an 18-gauge biopsy device (ASAP 18, Microvasive-. Eighty-three biopsies were performed by an experienced radiologist and 47 by several less experienced radiologists. All biopsies were guided by biplane fluoroscopy. RESULTS: We successfully obtained sufficient tissue(>2-mm in the length) in 128(985) of 130 biopsies. Biopsy provided the specific diagnosis in 97 (85%) of 114 patients including 78 (88%) of 89 patients with a malignant tumor and 19 (90%) of 21 patients with a benign condition. The diagnosis could not be made in the remaining four patients. Of interest to note was the superb capability (74/74) of biopsy to make a distinction between small cell carcinoma and non-small cell carcinoma. There was no significant difference in the diagnostic yields between the experienced and less experienced radiologists, Of the total 130 biopsies, pneumothorax appeared in 13 (10%), among which treatment was required in 2 (2%), Mild, self-limiting hemoptysis was noted in seven (5%), but in no case was the treatment required. CONCLUSION: We conclude transthoracic lung biopsy with an 18-gauge automated device is an effective procedure for the specific diagnosis of benign and malignant lung disease. It is safe with the complication rate cornparable to that of fine-needle aspiration biopsy as well.