Background:Oestrogen receptor (ER)- and progesterone receptor (PR)-negative (ER−PR−) breast cancer is associated with poorer prognosis compared with other breast cancer subtypes. High parity has been associated with an increased risk of ER−PR− cancer, but emerging evidence suggests that breastfeeding may reduce this risk. Whether this potential breastfeeding benefit extends to women at high risk of breast cancer remains critical to understand for prevention.Methods:Using population-based ascertained cases (n=4011) and controls (2997) from the Breast Cancer Family Registry, we examined reproductive risk factors in relation to ER and PR status.Results:High parity (3 live births) without breastfeeding was positively associated only with ER−PR− tumours (odds ratio (OR)=1.57, 95% confidence interval (CI), 1.10-2.24); there was no association with parity in women who breastfed (OR=0.93, 95% CI 0.71-1.22). Across all race/ethnicities, associations for ER−PR− cancer were higher among women who did not breastfeed than among women who did. Oral contraceptive (OC) use before 1975 was associated with an increased risk of ER−PR− cancer only (OR=1.32, 95% CI 1.04-1.67). For women who began OC use in 1975 or later there was no increased risk.Conclusions:Our findings support that there are modifiable factors for ER−PR− breast cancer and that breastfeeding in particular may mitigate the increased risk of ER−PR− cancers seen from multiparity. [ABSTRACT FROM AUTHOR]