Reproductive factors have been shown to affect pre- and postmenopausal breast cancer risk differently, but whether there are additional age-specific differences among menopausal women as they age has not been clarified. We analyzed data from a large population-based case-control study that included 1,508 breast cancer cases and 1,556 controls, aged 20-98 years, who completed an in-home interviewer-administered questionnaire. The following subgroups were created to examine if the associations between reproductive factors and breast cancer risk varied by age- and menopausal-status: premenopausal (n=968), postmenopausal <65 years (n=1,045), postmenopausal >or=65 years (n=958). Among postmenopausal women >or=65 years, ever having breastfed decreased risk (odds ratio (OR)=0.67, 95% confidence interval (CI)=0.48, 0.92), and a strong dose-response relationship was observed for longer durations of breastfeeding (P trend=0.02), with the most pronounced protective effect observed for >or=14 months of breastfeeding (OR=0.40, 95% CI=0.21,0.76). Late age at first birth (AFB) and older age at last birth (ALB) were associated with non-statistically significant increases in breast cancer risk in this older group, while late age at menarche and surgical menopause decreased risk. ORs for multiparity were close to the null. Among premenopausal women and postmenopausal women <65 years, multiparity significantly decreased risk, and older AFB nonsignificantly increased risk. Our findings suggest that the well-known protective effect of multiparity attenuates with older age. Moreover, breastfeeding, one of the few potentially modifiable risk factors for breast cancer, was an important factor in decreasing risk among older parous postmenopausal women.