As part of an international study, an attempt was made to identify all new cases of carcinoma of the breast diagnosed in white female residents of the city of Boston, Massachusetts, and selected suburbs between January 1, 1965, and December 31, 1966; 758 cases were identified, giving an average annual rate of 86.5/100,000 female population. Rates by age and marital status are reported. Of these patients, 606 were interviewed, together with 1,807 patients in the same hospitals with diseases other than cancer of the breast. As expected, relative to the comparison group the breast cancer cases were of low parity. Age at first pregnancy was an important risk factor, the estimated risk for women reporting first pregnancies at age 30 or older being about double that for women with first pregnancy under 20. This association was independent of the tendency of breast cancer patients to have fewer children. No consistent difference was found between the breast cancer cases and the comparison group in any of a variety of measures of lactation experience. In particular, after allowance for their low parity, the cases did not reveal any deficit of women with histories of prolonged lactation. Neither were differences in lactation experience evident when consideration was restricted to the first pregnancy or to pregnancies at an early age. By comparison with the other hospital patients, the breast cancer patients had fewer than expected women who had had surgical menopauses before 35 years of age. The breast cancer cases did not differ from the comparison group in reported age at menarche or age at natural menopause.