To evaluate the mammographic and histopathologic features of carcinomas not diagnosed at stereotactic core biopsy. A retrospective review revealed 144 surgically confirmed carcinomas preoperatively sampled with stereotactic core biopsy. Diagnosis at stereotactic core biopsy was carcinoma in 116 (81%) lesions, atypical hyperplasia in 21 (15%), and benign findings discordant with those from mammography in seven (5%). Mammographic and histopathologic findings in the latter 28 cases were reviewed. Prompt repeat biopsy was recommended in all 28 cases. The frequency with which a cancer yielded atypical hyperplasia at stereotactic core biopsy was higher for calcifications than masses (30% vs 5%, P < .0001), ductal carcinoma in situ (DCIS) than infiltrating carcinoma (33% vs 7%, P = .0002), and noncomedo than comedo DCIS (60% vs 9%, P = .0008). No significant difference was observed in the likelihood of benign core biopsy findings without atypia in malignant calcifications versus masses (7% vs 3%, P = .43), DCIS versus infiltrating carcinoma (7% vs 4%, P = .43), or noncomedo versus comedo DCIS (0% vs 9%, P = .49). The likelihood of not diagnosing carcinoma was highest for calcifications and for noncomedo DCIS. Discordance in mammographic and histopathologic findings or the presence of atypical hyperplasia may enable the radiologist to identify missed or underestimated carcinomas prospectively and avoid a delay in diagnosis.