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The possibility of a relation between breast-feeding and breast cancer was raised some 70 years ago when both the inability to lactate and prolonged lactation were more often noted from the medical histories of women with breast cancer. Investigators, combining data from seven countries in the international collaborative case-control study 25 years ago, found no overall relation between breast-feeding and breast cancer. This finding led to the belief that any such apparent relationship was spurious and due to the confounding effects of parity. Several studies, published in the late 1960s and in the 1970s, did not show convincing associations but suggested a somewhat lower risk among women below the age of 50 who had breastfed.In a retrospective analysis in the Nurses' Health Study, a large cohort of North American women, no important association between breast-feeding reported in 1986, and breast cancer incidence between 1976 and 1986, was seen.As a cross-check against possible differential bias arising from inaccurate recollection of breast-feeding history among women with and without breast cancer, the present authors conducted a prospective analysis among women in the Nurses' Health Study who were free of cancer in 1986.In 1976, 121,701 female nurses, aged 30 to 55 years, living in 11 states, completed a self-administered questionnaire that included items such as risk factors for cancer and heart disease. Information was collected on demographic and reproductive factors as well as on family and personal history of cancer. Every 2 years, follow-up questionnaires have been mailed to cohort members to update the information on potential risk factors and to identify newly diagnosed cases of cancer and other medical events.Among the 89,887 parous women who reported on breast-feeding history in 1986, 37 percent reported no breast-feeding, 11 percent reported less than 1 month, 12 percent between 1 and 3 months, 12 percent between 4 and 6 months, 10 percent between 7 and 11 months, 12 percent between 12 and 23 months, 5 percent between 24 and 47 months, and 1 percent 48 months or more. Between 1986 and 1992, 1459 incident cases of invasive breast cancer were identified among the women included in the analysis.No important overall association was found between a history of ever having breast-fed and breast cancer incidence. The association that was found did not change subsequently with adjustment for parity, age at first birth, age at menarche, age at menopause, postmenopausal hormone use, family history of breat cancer, history of benign breast disease, body mass index (BMI), use of oral contraceptives, alcohol intake, vitamin A intake (including supplements), or physical activity in a multivariate logistic regression model. Adjusting for age in 1-year categories did not change the estimates.When cumulative duration of breast-feeding for all births was considered, no important trend emerged. The adjusted relative risk (RR) for women who reported having breast-fed for 24 months or more, as compared with parous women who had never lactated, was 1.11 (0.9-1.38). Women who breast-fed for 48 months or more had RR for breast cancer of 1.39 (0.87-1.50).Among premenopausal women, the RR associated with a history of having ever breast-fed, adjusted for known determinants of breast cancer, was 1.14 (0.87-1.50). Among postmenopausal women, the RR was 0.89 (0.79-1.00). Risk did not vary appreciably by duration of breastfeeding among either premenopausal or postmenopausal women.The association between breast-feeding and breast cancer incidence varied by parity. When the analysis was restricted to women who had given birth to one child only, the RR was 0.68 (0.46-1.00) for women who had ever lactated, as compared with those who had not.