To assess the relation of smoking cessation to the risk of a first myocardial infarction in women, we compared the smoking habits of 910 patients who had had their first myocardial infarction with those of 2375 controls in a hospital-based case-control study of women from 25 to 64 years of age. The estimate of relative risk among current smokers as compared with women who had never smoked was 3.6 (95 percent confidence interval, 3.0 to 4.4). Among exsmokers overall, the corresponding estimate of relative risk was 1.2 (95 percent confidence interval, 1.0 to 1.7). Among exsmokers, the estimate of relative risk was significantly elevated among women who had stopped smoking less than two years previously (relative risk, 2.6; 95 percent confidence interval, 1.8 to 3.8). Most of the increase in the risk had dissipated among the women who had stopped smoking two to three years previously, and the estimate of relative risk among the women who had not smoked for three or more years was virtually indistinguishable from that among the women who had never smoked. The same pattern of decline was apparent regardless of the amount smoked, the duration of smoking, the age of the women, or the presence of other predisposing factors. These data suggest that in women, as in men, the increase in the risk of a first myocardial infarction among cigarette smokers declines soon after the cessation of smoking and is largely dissipated after two or three years.