Associations between parity and the risk of breast cancer, and the relative importance of age at first and age at last birth on breast cancer risk, were estimated in a case-control study nested in a nation-wide cohort of Swedish women born between 1925 and 1960. A total of 12,782 women with breast cancer and five times as many individually age-matched controls, aged less than 60 years with concomitant fertility information, were included in the analysis. Increasing parity was associated with a pronounced decrease in the risk of breast cancer with each additional birth conferring a 10 percent risk reduction (odds ratio 0.90 [95% CI 0.88-0.91]). In an analysis limited to women with two or more parities, and after adjustment for the effects of ages at interim births, the risk of breast cancer increased by about 13 percent for each five-year increment in age at first birth (odds ratio 1.13 [1.08-1.19]). For every five year-increase in age at last birth there was a small risk increase of marginal statistical significance (odds ratio 1.05 [1.01-1.09]). The present findings contradict recent claims that age at last birth has a stronger effect than age at first birth on breast cancer risk. The dominance of age at first birth as risk modulator is likely to reflect the protection afforded by the terminal differentiation of breast cells induced by a first pregnancy.