The role of the major identified risk factors for breast cancer was assessed using data from a hospital-based case-control study conducted in Northern Italy on 1108 women with histologically confirmed breast cancer and 1281 control subjects with a spectrum of acute conditions unrelated to any of the established or potential risk factors for breast disease. With reference to nulliparous women, the risk of breast cancer was below unity for those who first gave birth below age 25, and above unity for those with later first full-term pregnancy. However, in each stratum of age at first birth, the point estimate was below unity for women with five or more births. The relative risk for greater than or equal to 5 births compared with 1 or 2 was 0.6 (95% confidence interval = 0.4-0.9) when allowance was made for age at first birth. Likewise, there was a significant and independent effect of age at last birth which was evident in various strata of parity and age at first birth. The overall relative risk for last birth at 30 years or over compared with under 30 was 1.4 (95% confidence interval = 1.1-1.8). There was little relation of breast cancer risk with abortions or miscarriages. Breast cancer cases reported earlier menarche and later menopause; further, lifelong irregularities in menstrual pattern were less common among the cases (relative risk = 0.6, 95% confidence interval = 0.5-0.8). The risk estimates were elevated in women with positive history of benign breast disease, family history of breast cancer and greater body mass index.(ABSTRACT TRUNCATED AT 250 WORDS)