OBJECTIVE To determine whether histologic findings of cancer or atypical hyperplasia at large-core needle biopsy (LCNB) of nonpalpable breast lesions match histologic findings at excision. METHODS Stereotaxic LCNB was performed with an automated prone unit, biopsy gun, and 14-gauge cutting needles in 450 nonpalpable breast lesions. Lesions classified as carcinoma or atypical ductal hyperplasia (ADH) at histologic examination after LCNB were excised. A pathologist retrospectively compared core and excisional histologic findings. RESULTS Histologic comparison was performed in 116 of 135 carcinomas after LCNB. Histologic findings were concordant in 99 carcinomas. Partial discordance in 17 carcinomas led to an additional surgical procedure in one case. Histologic comparison was performed in 16 of 19 ADHs diagnosed with LCNB. Histologic findings were concordant in five and discordant in 11 ADHs. CONCLUSIONS LCNB findings of carcinoma are accurate and allow definitive therapeutic surgery, including mastectomy. LCNB findings of ADH are inaccurate, and excisional biopsy is necessary.