Purpose: The aim of this article was to investigate the efficacy of ultrasonography-guided core needle biopsy and prognostic factor analysis of breast cancer to plan overall treatment strategy. Patients and Methods: A consecutive series of nonpalpable and palpable breast cancers constituted our study group (n= 201 lesions; mean size, 20.4 mm) Mean number of core samples was 3.4. Malignant lesions diagnosed with core biopsy underwent therapeutic surgical excision. Core biopsy and surgical excisions were compared for histologic type, grade, estrogen receptors (ERs), progesterone receptors (PgRs), and c-erbB2 levels. Cutoff values for ER, PgR, and c-erbB2 affecting the management strategy were selected as 10%, 10%, and 50%, respectively. Results: Eighty-five lesions (42.3%) were malignant in core biopsy (mean size, 18.4 mm). 1 Among these, 11 were inoperable and 13 were surgically excised at other institutions. In 61 lesions, core and surgical excision specimens were evaluated in the same institution (mean tumor size, 18.6 mm; range 6-60 mm). Concordance between the 2 biopsy methods was 85.2% (52 of 61) for histologic type of tumor, 1 68.8% (33 of 48) for tumor grade, 90% (27 of 30) for ER, 86.7% (26 of 30) for PgR, and 79.3% (23 of 29) for c-erbB2 levels. Appropriate site selection for sampling was indicated to be of paramount importance, I especially in determining reliable ER, PgR, and c-erbB2 levels. Conclusion: Core needle biopsy of breast cancer is equally effective compared wth surgical biopsy and can be used in overall treatment planning. However, appropriate site selection for sampling should be guaranteed using ultrasonographic guidance.