Abstract Background: Screening for disease in healthy people inevitably leads to some false-positive tests. A recent study (von Euler-Chelpin et al, JNCI 2012) showed that women with a false-positive test from screening with mammography had an increased risk of being diagnosed with breast cancer later in life as compared to women with only negative results (RR = 1.67, 95% CI 1.45 to 1.88). The reasons for this excess risk remain unknown. In the present study we have assessed misclassification and evaluated the impact of misclassification on over all excess risk. Method: We used data from the population-based screening mammography program in Copenhagen, Denmark, and studied 295 women exposed to a false-positive test that later developed breast cancer. Screening took place between 1991 and 2006, with a follow-up for cancer until 2008. Cancers that developed in the same location as the finding that initially caused the recall were defined as misclassified and excluded in the assessment of the excess risk among the false-positives. Time from diagnosis, breast density, tumor size, receptor and nodal status were also taken into account. Results: 72 cases were found to be misclassified. When the misclassified tests had been excluded, we still found a significant excess risk for breast cancer of 27% (RR = 1.27, 95% CI (1.11 to 1.46) among the women with a false-positive test compared to women with only negative tests. Women with a false-positive test proven at assessment (Type 1) had an excess risk of 27% (RR = 1.27, 95% CI (1.09 to 1.46), while false-positives proven at surgery (Type 2) had an excess risk of 30 % (RR = 1.30, 95% CI (0.86 to 1.96). In the group of women with misclassified tests there were a significantly higher proportion of breasts with fatty tissue (BI-RADS density classification 1 or 2) than among the non-misclassified. The average time from first false-positive test to time of diagnosis was significantly shorter among women with misclassified tests, 4.0 years versus 7.8 years (p < .0001). There were no significant differences in tumor size, receptor or nodal status. Conclusion: The results indicate that the increased risk can not be explained only by misclassification. The excess risk remains for both Type1 and Type 2 false-positives, which favors some biological susceptibility, as women with false-positive test Type 2 have undergone surgery, which thus makes residual misclassification unlikely. Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P3-02-03.