BACKGROUND: Magnetic resonance imaging (MRI) has been used to supplement screening mammography and clinical breast examination (CBE) in women who are at high risk of developing breast cancer. In this study, the authors investigated the efficacy of alternating screening mammography and breast MRI every 6 months in women who had a genetically high risk of developing breast cancer. METHODS: A retrospective chart review was performed on all women who were seen in a high-risk breast cancer clinic from 1997 to 2009. Patients with breast cancer gene (BRCA) mutations who underwent alternating screening mammography and breast MRI every 6 months were included in the study. Mammography, ultrasonography, MRI, and biopsy results were reviewed. RESULTS: Of 73 patients who met the study criteria, 37 had BRCA1 mutations, and 36 had BRCA2 mutations. Twenty-one patients (29%) completed 1 cycle of mammography and MRI surveillance, 23 patients (31%) completed 2 cycles, 18 patients (25%) completed 3 cycles, and patients 11 (15%) completed !4 cycles. The median follow-up was 2 years (range, 1-6 years). Thirteen cancers were detected among 11 women (15%). The mean tumor size was 14 mm (range, 1-30 mm), and 2 patients had bilateral cancers. Twelve of 13 cancers were detected on an MRI but not on the screening mammography study that was obtained 6 months earlier. One cancer (a 1-mm focus of ductal carcinoma in situ) was an incidental finding in a prophylactic mastectomy specimen. One patient had ipsilateral axillary lymphadenopathy identified on ultrasonography but had no evidence of lymph node involvement after neoadjuvant chemotherapy and surgery. CONCLUSIONS: In women who were at genetically high risk of developing breast cancer, MRI detected cancers that were not identified on mammography 6 months earlier. Future prospective studies are needed to evaluate the benefits of this screening regimen.