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Background Although childbearing is known to protect against breast cancer, whether or not breastfeeding contributes to this protective effect is unclear. Methods Individual data from 47 epidemiological studies in 30 countries that included information on breastfeeding patterns and other aspects of childbearing were collected, checked, and analysed centrally, for 50 302 women with invasive breast cancer and 96 973 controls. Estimates of the relative risk for breast cancer associated with breastfeeding in parous women were obtained after stratification by fine divisions of age, parity, and women’s ages when their first child was born, as well as by study and menopausal status. Findings Women with breast cancer had, on average, fewer births than did controls (2·2 vs 2·6). Furthermore, fewer parous women with cancer than parous controls had ever breastfed (71 % vs 79%), and their average lifetime duration of breastfeeding was shorter (9·8 vs 15·6 months). The relative risk of breast cancer decreased by 4·3 % (95 % CI 2·9–5·8; p<0·0001) for every 12 months of breastfeeding in addition to a decrease of 7·0 % (5·0–9·0; p<0·0001) for each birth. The size of the decline in the relative risk of breast cancer associated with breastfeeding did not differ significantly for women in developed and developing countries, and did not vary significantly by age, menopausal status, ethnic origin, the number of births a woman had, her age when her first child was born, or any of nine other personal characteristics examined. It is estimated that the cumulative incidence of breast cancer in developed countries would be reduced by more than half, from 6·3 to 2·7 per 100 women by age 70, if women had the average number of births and lifetime duration of breastfeeding that had been prevalent in developing countries until recently. Breastfeeding could account for almost two-thirds of this estimated reduction in breast cancer incidence. Interpretation The longer women breast feed the more they are protected against breast cancer. The lack of or short lifetime duration of breastfeeding typical of women in developed countries makes a major contribution to the high incidence of breast cancer in these countries