Purpose To determine additional cancer yield of magnetic resonance (MR) imaging in women with breast cancer detected at screening ultrasonography (US) and to identify a subgroup of women who are likely to benefit from preoperative MR imaging. Materials and Methods This study was approved by the institutional review board, and the requirement for informed consent was waived. A retrospective review of 374 women (median age, 48 years; age range, 30-74 years) with breast cancer detected at screening US (invasive, n = 321) who underwent preoperative breast MR imaging between 2007 and 2013 was performed. Cancer yield and positive predictive value of biopsy were calculated. Multivariate logistic regression analysis was performed to identify clinical-pathologic features associated with additional cancer detected at MR imaging. Results Of 374 women, 21 (5.6%; 95% confidence interval [CI]: 3.5%, 8.5%) were diagnosed with additional cancer (positive predictive value of biopsy, 42.0% [21 of 50 women]; 95% CI: 28%, 57%). Index invasive lobular cancer (ILC) histologic type was significantly associated with additional cancer detected at MR imaging (odds ratio, 4.0; 95% CI: 1.2, 13.6; P = .03). In women with index invasive cancer, premenopausal status (odds ratio, 5.7; 95% CI: 1.2, 35.8; P = .03) and lobular histologic type (odds ratio, 3.9; 95% CI: 1.1, 12.3; P = .03) were factors associated with additional cancer detected at MR imaging. Conclusion Preoperative MR imaging helped to detect additional sites of cancer in 5.6% of women with breast cancer detected at screening US. Women with index ILC and premenopausal women are more likely to benefit from preoperative MR imaging. © RSNA, 2016 Online supplemental material is available for this article.