OBJECTIVE: We assessed the relation between duration of lactation and maternal incident myocardial infarction. STUDY DESIGN: This was a prospective cohort study of 89,326 parous women in the Nurses' Health Study. RESULTS: During 1,350,965 person-years of follow-up, 2540 cases of coronary heart disease were diagnosed. Compared with parous women who had never breastfed, women who had breastfed for a lifetime total of 2 years or longer had 37% lower risk of coronary heart disease (95% confidence interval, 23-49%; P for trend < .001), adjusting for age, parity, and stillbirth history. With additional adjustment for early-adult adiposity, parental history, and lifestyle factors, women who had breastfed for a lifetime total of 2 years or longer had a 23% lower risk of coronary heart disease (95% confidence interval, 6-38%; P for trend = .02) than women who had never breastfed. CONCLUSION: In a large, prospective cohort, long duration of lactation was associated with a reduced risk of coronary heart disease.