A case-control study was conducted over a period of 11 months in an area containing one-third of the Swedish population. One thousand and one patients participated, constituting 94% of all women newly diagnosed as having breast cancer within the area. They were compared with 1,001 age-matched, non-hospitalized controls without breast cancer, selected by paired sampling from a population register. The risk of breast cancer was slightly, but significantly, related to parity, the standardized relative risk (SRR) being 1.35 for nulliparous women as compared to ever parous. In the different parity groups a risk significantly lower than that for nulliparous women was found only for women with more than 2 children (SRR = 0.59) but the trend with parity was highly significant (P less than 0.001). Age at first birth was not found to be an important risk factor for breast cancer. SRR was lower than for nulliparous women in all groups of women with their first birth before the age of 35 years, but the difference was significant (P less than 0.05) only for those with the first birth between 20 and 24 (SSR = 0.69) and 25 and 29 (SRR = 0.69) years of age. The trend with age at first birth (P less than 0.05) disappeared after stratification for parity, suggesting that it was a confounding factor.