Estimates of the prevalence of atrial fibrillation (AF) in patients with coronary artery disease have varied from "frequent" to less than 2%. Data on 18,343 patients with angiographically demonstrated CAD in the Coronary Artery Surgery Study (CASS) registry were reviewed and AF was found to be present in 116 (0.6%) patients. The presence of AF was positively associated with the following clinical and angiographic variables: older age, sex (male), mitral regurgitation and functional impairment due to congestive heart failure. The number of diseased coronary arteries was negatively related to the presence of AF. Atrial fibrillation was an independent predictor of survival and its presence doubled the estimated risk over those patients without AF.