Purpose: Invasive ductal carcinoma (IDC) is diagnosed with or without a ductal carcinoma in situ (DCIS) component. Previous analyses have found significant differences in tumor characteristics between pure IDC lacking DCIS and mixed IDC with DCIS. We will test our hypothesis that pure IDC represents a form of breast cancer with etiology and risk factors distinct from mixed IDC/DCIS. Methods: We compared reproductive risk factors for breast cancer risk, as well as family and smoking history between 831 women with mixed IDC/DCIS (n = 650) or pure IDC (n = 181), and 1,620 controls, in the context of the Women’s Circle of Health Study (WCHS), a case–control study of breast cancer in African-American and European-American women. Data on reproductive and lifestyle factors were collected during interviews, and tumor characteristics were abstracted from pathology reports. Case–control and case–case analyses were conducted using unconditional logistic regression. Results: Most risk factors were similarly associated with pure IDC and mixed IDC/DCIS. However, among postmenopausal women, risk of pure IDC was lower in women with body mass index (BMI) 25 to <30 [odds ratio (OR) 0.66; 95 % confidence interval (CI) 0.35–1.23] and BMI ≥ 30 (OR 0.33; 95 % CI 0.18–0.67) compared to women with BMI < 25, with no associations with mixed IDC/DCIS. In case–case analyses, women who breastfed up to 12 months (OR 0.55; 95 % CI 0.32–0.94) or longer (OR 0.47; 95 % CI 0.26–0.87) showed decreased odds of pure IDC than mixed IDC/DCIS compared to those who did not breastfeed. Conclusions: Associations with some breast cancer risk factors differed between mixed IDC/DCIS and pure IDC, potentially suggesting differential developmental pathways. These findings, if confirmed in a larger study, will provide a better understanding of the developmental patterns of breast cancer and the influence of modifiable risk factors, which in turn could lead to better preventive measures for pure IDC, which have worse disease prognosis compared to mixed IDC/DCIS.