Purpose : To evaluate the efficacy of stereotaxic breast core biopsy using a prone table system, and theeffects of operator experience, lesion characteristics and number of samples on biopsy results in cases involvingnonpalpable breast lesions. Materials and Methods : We performed stereotaxic core biopsies of 62 nonpalpablemammographic lesions in 61 patients. Subsequent surgical excision was performed in 11 cases withmicrocalcifications and one case with a mass. We equally divided patients with microcalcifications into two groups(early and late periods) and analyzed the mammographic findings. Correlation of the pathologic results of corebiopsy with those of surgical excision were investigated. Results : In two patients, stereotaxic biopsy wasimpossible due to poor visibility of microcalcifications and thinness of the compressed breast. In 59 patients,core biopsy was successfully performed and specimens were adequate for pathologic examination. The average numberof microcalcifications seen on specimen mammography in the two groups was 1.8 (range: 0~8) and 2.5 (range:0~4)respectively. In patients from whom less than five and five or more samples were taken, the average number ofmicrocalcifications seen on specimen mammography was 1.5 (range: 0~6) and 2.6 (range: 0~8), respectively,throughout the whole period. The pathologic findings were fibrocystic change in 50 cases, fibroadenoma in four,ductal carcinoma in situ in four, invasive ductal carcinoma in one, and atypical ductal hyperplasia in one. Theagreement rate of pathologic results between core biopsy and surgical excision was 83% (10/12) for malignancy and75% for histology. In three cases with disagreement between core and surgical pathologic results, the samplingnumber was small (3~4 times) and in two of the three cases, microcalcifications were not visible on mammography.Conclusion : Operator experience and sampling numbers larger than five results in an increased number ofmicrocalcifications in specimens and more reliable core biopsy.