Purpose To determine whether human epidermal growth factor receptor 2 (HER2) –positive status is associated with risk of breast cancer diagnosis in the interval between mammographic screening, we estimated the distribution of features of aggressive tumor behavior in a general population with newly diagnosed breast cancer and known screening status. Patients and Methods We evaluated all invasive breast cancers (N = 641) that were systematically collected by the Parma Province Cancer Registry and diagnosed in women age 50 to 69 years from 2004 to 2007. From this population, 292 screen-detected cancers and 48 interval cases with negative screening mammograms on expert rereading (true interval cancers) were selected for study purposes. Unconditional logistic regression adjusted for age and tumor size was used to determine whether interval cancers were associated with selected clinicobiologic characteristics. Results Tumors with a high histologic grade (odds ratio [OR], 1.8; 95% CI, 1.2 to 3.8), high proliferative rate (OR, 2.4; 95% CI, 1.2 to 4.5), negative estrogen receptor status (OR, 1.6; 95% CI, 1.1 to 3.1), or HER2-positive status (OR, 3.4; 95% CI, 1.7 to 7.1) were more likely to be diagnosed in the interval between screening. Women age less than 60 years with HER2-positive breast cancer were four times more likely to be diagnosed in the interval between screening compared with only a two-fold increased risk for older women. Conclusion This population-based cancer registry study demonstrated that HER2-positive tumors account for a substantial proportion of mammographic screening failure. The distribution of biologic characteristics in screen-detected cancers differs from that observed in interval cancers and may account in part for the more aggressive behavior of interval-detected cases.