Purpose : To evaluate the clinical usefulness of ultrasonography(US) - guided automated gun biopsy ofnonpalpable breast lesions. Materials and Methods : In 30 nonpalpable breast lesions over 0.6cm and detected onUS, we performed US-guided biopsy using an 18-gauge automated biopsy gun. Two to four specimens were obtained fromeach lesion. We analyzed the site, size and depth of the lesions, and the length and histopathologic results ofthe specimens. In four lesions, surgical biopsy and gun biopsy results were compared. Results : In 29 of 30lesions(96.7%), specimens were adequate for histopathologic diagnosis, and this was as follows: one case ofinfiltrating ductal carcinoma, 13 of fibrocystic disease, 10 of fibrocystic disease versus fibroadenoma and one offibroadenoma. 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 minorbleeding, which was controlled by compression. Conclusion : US-guided automated gun biopsy is a clinically usefuland safe procedure for evaluating nonpalpable breast lesions detected on US.