BACKGROUND AND OBJECTIVES: To examine the use of additional imaging after standard mammographic screening views to better understand the value of these additional testing in improving accuracy. MATERIALS AND METHODS: Statewide population data on screening mammography were used to report accuracy measures at screening and after additional imaging tests. Pathology data from biopsies performed within 1 year following the screening mammogram were used to determine cancer outcome (489 in situ and invasive cancers). Pathologic and population characteristics of women receiving different types of imaging were assessed by chi-square or t-tests. Similar tests compared women with the same imaging and differing outcomes. RESULTS: Of 77,799 women with screening mammograms 9.9% had additional imaging. Additional imaging reduced false positives from 7,765 (100/1,000 mammograms) to 1,112 (14/1,000 mammograms). The majority of false negatives (82%) occurred in women receiving only screening views, and additional imaging increased the number of false negatives from 82 (1/1,000 mammograms) to 115 (1.5/1,000 mammograms). CONCLUSION: Additional imaging can reduce unnecessary biopsy but at the cost of some additional false negatives. Additional imaging's potential for improving the sensitivity of screening is limited because most missed cancers occur in women who do not have additional imaging.