Background: Women with breast cancer increasingly choose to undergo contralateral prophylactic mastectomy (CPM) even as the benefit of this procedure for woman at average risk for breast cancer remains uncertain. Many women with newly diagnosed breast cancer undergo pre-operative MRI for a variety of indications. Growing evidence suggests that obtaining a pre-operative MRI increases the likelihood that a patient will choose CPM. This study evaluates the relationship between a pre-operative MRI and the decision to pursue CPM, as well as the rate of contralateral MRI findings for which follow-up is recommended and the choice to undergo CPM. The pathology found in contralateral breasts in this series is also reported. Methods: Newly diagnosed breast cancer patients were prospectively enrolled in Breast Molecular Epidemiology Resource (B-MER) observational study at the University of Iowa from April 2010 through March 2013. Prophylactic mastectomy is defined as removal of the contralateral breast within 12 months of definitive mastectomy. Univariate logistic regression was used to identify factors predictive of undergoing CPM. Recommended follow-up of the contralateral breast MRI is defined as any imaging or procedure other than immediate ultrasound evaluation. Results: Among 134 patients (mean age 53) who underwent mastectomy, 53 (40%) underwent CPM. Univariate analysis revealed that patients undergoing CPM were more likely to have had a preoperative bilateral MRI (52%% vs. 28%, p = 0.006) and were more likely to have been given a recommendation for a follow-up test (79% vs, 40%, p = 0.007). Univariate Analysis of Odds of Electing to Undergo CPMVariableLevelOdds ratioP-valueAge5 year0.62<.001Breast MRIYes vs No2.740.006MRI follow-up recommendationYes or No5.530.007Nodal statusPositive vs Negative0.820.581History of benign breast biopsiesNo vs Yes1.850.209Family history of breast cancerYes vs No1.160.711ER/PR statusNegative vs Positive1.830.134Triple NegativeNo vs Yes1.910.196HER2 statusPositive vs Negative2.470.066BRCA testing doneYes vs No6.04<.001BRCA results*Positive vs Negative3.120.315Tobacco everYes or No1.390.354Alcohol everYes or No1.250.544* Indeterminate and not done levels were excluded from the analysis Univariate analysis also revealed associations between choice of CPM and younger age (p<0.001) and BRCA testing (p<0.001). In this series, CPM was not associated with nodal status, ER/PR status, history of benign breast biopsies, family history of breast cancer, BRCA result and tobacco or alcohol use, although there was a trend for association with HER2 status. Of the 53 patients who underwent CPM, one had proliferative disease with atypia, 34 had proliferative disease without atypia. A single patient had DCIS found in the contralateral breast which had not been identified on MRI. Conclusions: The use of preoperative breast MRI and abnormal imaging findings in the contralateral breast, for which follow-up was recommended, led to a higher CPM rate. Contralateral prophylactic mastectomy rarely uncovered occult malignancy. Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P2-02-07.