The relationship between socio-economic indicators, body mass, reproductive and menstrual factors and risk of "familial" breast cancer has been analyzed using data from a large case-control study of breast cancer conducted in Italy. A total of 3,037 breast-cancer patients aged 75 years or less admitted to a network of hospitals in the Greater Milan area were interviewed. Controls were 2,569 women aged 75 or less, admitted for acute conditions to the same network of hospitals where cases had been identified. Women were not included as controls if they had been admitted for gynecological, hormonal or neoplastic diseases. A total of 331 cases and 121 controls reported a family history of breast cancer in their first-degree relatives, with a family history tended to be more educated and of higher social class than controls with family history, but these differences were not statistically significant. No relationship emerged with marital status and body-mass index. The estimated multivariate relative risks were, compared with nulliparous women, respectively 0.9, 1.2 and 1.2 for women reporting 1, 2 and 3 or more births. The risk of breast cancer was also greater in women reporting one or more spontaneous abortions: compared with those who did not, the estimated relative risk was 1.9 (95% confidence interval 0.8 to 3.7). Relative to women reporting their first birth below age 25, the estimated relative risks were 1.3 and 0.9 in those reporting it at age 25 to 29 and 30 or more respectively. Likewise, no consistent relationship emerged with lifelong menstrual pattern and menopausal status. These results suggest that menstrual and reproductive factors do not play an important role in the risk of breast cancer in women with a history of the disease in first-degree relatives.