One hundred women, each with a single nonpalpable breast lesion evident at mammography, underwent computer-guided sterotaxic 14-gauge needle biopsy followed by hook-wire localization and open surgical biopsy. Lesions were not less than 5 mm in diameter. Core biopsy yielded essentially correct findings in 96 cases and in 35 of 36 cancers. Open surgery yielded the correct findings in 97 cases and also in 35 of 36 cancers. Two fibroadenomas were missed at core biopsy and one was missed at surgical biopsy. There was complete or partial histologic agreement in 94 cases. This is further evidence that in selected cases, stereotaxic core biopsy can be as accurate as open surgical biopsy.