We investigated the effects of cigarette smoking and cessation of smoking in a cohort of 1893 men and women from the Coronary Artery Surgery Study (CASS) registry who were 55 years of age or older and had angiographically documented coronary artery disease. The six-year mortality rate was greater among continuing smokers (n = 1086) than among those who quit smoking during the year before enrollment in the study and abstained throughout the study (n = 807) (relative risk, 1.7 [95 percent confidence limits, 1.4, 2.0]). Continuing smokers were also at higher risk of either myocardial infarction or death (1.5 [1.2, 1.7]). There was no diminution of the beneficial effect with increasing age. The relative risks of death were 1.7 (1.4, 2.1) and 1.6 (1.1, 2.3) for the groups 55 to 64 years old and 65 or older, respectively, and 1.6 (1.4, 1.9) for comparable subgroups among CASS subjects 35 to 54 years of age. When subjects were arrayed according to risk quartile, the benefits of smoking cessation were greatest in those at moderate risk. We conclude that smoking cessation lessens the risk of death or myocardial infarction in older as well as younger persons with coronary artery disease.