Background: We examined prospectively the relation of time since stopping smoking with risk of coronary heart disease in middle-aged women. Methods: The study was based on 12 years' follow-up data (1976 through 1988) from the Nurses' Health Study, an ongoing prospective cohort study, in which information on smoking habits was updated every 2 years by a mailed questionnaire. The study population consisted of 117 006 female registered nurses aged 30 to 55 years in 1976 who were free of coronary heart disease, stroke, and cancer at baseline. The outcome measure used was incident coronary heart disease, defined as nonfatal myocardial infarction plus fatal coronary heart disease. Results: A total of 970 incident cases of coronary heart disease (215 among participants who never smoked, 214 among former smokers, and 541 among current smokers) occurred during 1.37 million person- years of follow-up. The multivariate relative risk of total coronary heart disease among current smokers compared with participants who never smoked was 4.23 (95% confidence interval, 3.60 to 4.96). Risk of coronary heart disease was highest among smokers who started smoking before the age of 15 years (relative risk, 9.25; 95% confidence interval, 5.27 to 16.23). The relative risk among former smokers was 1.48 (95% confidence interval, 1.22 to 1.79). On stopping smoking, one third of the excess risk of coronary heart disease was eliminated within 2 years of cessation. Thereafter, the excess risk returned to the level of those who never smoked during the interval 10 to 14 years following cessation. Conclusion: Women who stop smoking will experience an immediate benefit as well as a further longer-term decline in excess risk of coronary heart disease to the level of those who never smoked.