Abstract Background: Estrogen receptor (ER)-negative breast tumors and progesterone receptor (PR)-negative breast tumors occur more commonly in women of African ancestry. Recent research indicates that the effects of reproductive factors may differ by hormone receptor status. We assessed the relation of parity and lactation to incidence of ER−/PR− and ER+/PR+ breast cancer in a cohort of African American women. Methods: From 1995–2009, 457 incident cases of ER+/PR+ and 318 cases of ER−/PR− breast cancer were confirmed by review of pathology data among 59,000 African American women followed in the Black Women's Health Study through biennial questionnaires. HRs and two-sided 95% CIs for the incidence of breast cancer subtypes were derived from proportional hazards regression models that controlled for age, reproductive variables, and breast cancer risk factors. Results: Higher parity was associated with an increased risk of ER−/PR− breast cancer (HR = 1.48, 95% CI: 0.98–1.84 for 3+ versus 0 births, Ptrend = 0.009), and with a reduced risk of ER+/PR+ cancer (HR = 0.53, 95% CI: 0.39–0.73 for 3+ versus 0 births, Ptrend = 0.0002). Among women who had breastfed, high parity was no longer associated with increased incidence of ER−/PR− breast, but the inverse association with ER+/PR+ cancer persisted. Conclusions: The higher incidence of ER−/PR− breast cancer in African American women may be explained in part by their higher parity and lower prevalence of breastfeeding relative to white women. Impact: Increased breastfeeding may lead to a reduction in the incidence of this breast cancer subtype. Cancer Epidemiol Biomarkers Prev; 20(9); 1883–91. ©2011 AACR.