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1508 Background: MRI has been accepted as a useful adjunct to screening mammography in high-risk women. Concerns remain over false positive findings, however, and little is known about harms and yield over time. Such information will help women decide about enhanced surveillance. Methods: Of 350 high risk patients offered MRI screening, 320 underwent 757 screens with a 1.5 Tesla magnet from 2008 to 2012 alternating (q 6 mo.) with digital mammography. Data collected included patient characteristics, mammographic density, estimated lifetime risk, and need for additional imaging and/or biopsy. Harms were defined as second look ultrasounds, US or MRI guided core biopsies, surgical biopsies or recommendation for short interval follow up. Estimates of harms over time were modeled by logistic regression. A significance level of 0.05 was used for all testing. Results: Compliance with MRI screening as recommended was 91% with the first MRI, and was not associated with age, race or level of risk. Harms were highest with the first MRI and decreased significantly with subsequent MRIs. Of 59 biopsies, 7 were malignant. Two were found in MRI 1, 3 in MRI 3 and 2 in MRI 4. Women with biopsies resulting from false positive findings were significantly younger (median age 44.5 as compared with 48; p=0.049) and were more likely to have extremely dense breast tissue (36% vs 17%; p=0.028.) Conclusions: This study of highly compliant high risk patients supports the use of MRI as an adjunct to mammography for early detection. The rate of harmful events decreased over time, yet cancer detection did not. This information is critical in counseling women who are considering annual screening breast MRI, particularly younger women and those with dense tissue. [Table: see text]