PURPOSE: To evaluate the clinical usefulness of ultrasonography (US) - guided automated gun biopsy of nonpalpable breast lesions. MATERIALS AND METHODS: In 30 nonpalpable breast lesions over 0.6cm and detected on US, we performed US-guided biopsy using an 18-gauge automated biopsy gun. Two to four specimens were obtained from each lesion. We analyzed the site, size and depth of the lesions, and the length and histopathologic results of the specimens. In four lesions, surgical biopsy and gun biopsy results were compared. RESULTS: In 29 of 30 lesions (96.7%), specimens were adequate for histopathologic diagnosis, and this was as follows: one case of infiltrating ductal carcinoma, 13 of fibrocystic disease, 10 of fibrocystic disease versus fibroadenoma and one of fibroadenoma. There was also one reactive hyperplasia of LN, and one fatty one and two normal tissues, and inthese four lesions, agreement between gun and surgical biopsy results was 100%. The only complication was minor bleeding, which was controlled by compression. CONCLUSION: US-guided automated gun biopsy is a clinically useful and safe procedure for evaluating nonpalpable breast lesions detected on US.