It is necessary to confirm disease pathologically even though there are several methods of diagnosis for breast abnormality, Ultrasound guided core needle biopsy (UGCNB) of breast lesions with automated gun is widely accepted as a simple and accurate procedure for avoiding open biopsy. From January 1996 to February 1998, 121 breast lesions of 117 women detected with mammogram, ultrasound, physical examination or any combination were selected. UGCNB was performed in all 121 cases with 7.5 MHz transducer and Manan Pro-mag 2.2 automated biopsy gun (2.2 cm excursion). After local anesthesia, 5-6 cores were obtained through 2mm skin incision site. Lesions categorized as probably benign to highly suspicious were included as indications for core biopsy, 67.5% of the 117 patients were in age thirties and forties. The results of the biopsy were benign in 65 lesions and malignant in 56. 15 patients showing benign results which were contrary to clinical findings and pathologic results, underwent open biopsy. Four patients who had previously been diagnosed as being benign, were found to be malignant (false-negative 3.3%). Primary results of four cases of false negative were intraductal papilloma, severe inflammatory change, atypical ductal hyperplasia, and insufficient sampling, respectively. In case of insufficient sampling resulted from small gauge needle (18G), correct diagnosis was possible in all lesions with large needle (14G). Also excisional biopsy was required in women having intraductal papilloma, severe inflammation and atypical ductal hyperplasia which were difficult to differenciate from malignancy. The UGCNB is excellent biopsy method for multiple breast lesions. The method has demonstrated a 96.7% sensitivity specificity 100%, and positive predictive value (PPV) of 46.3%. We also conclude that UGCNB is proved to be an useful alternative to surgical biopsy on simpler procedure with no adverse cosmetic results.