A total of 1666 patients undergoing isolated coronary artery bypass were studied for the occurrence of postoperative atrial fibrillation. Possible associations of this arrhythmia with various preoperative, intraoperative, and postoperative factors were studied by univariate (chi 2 and t tests) and multivariate (logistic regression) analyses. The overall incidence of postoperative atrial fibrillation was 28.4%, with the major occurrence 2 days after the operation. Both univariate and multivariate studies indicated the patient's age to be the dominant factor promoting postoperative atrial fibrillation, with an increasing prevalence in older patients (p = 0.0001). Multivariate analysis showed that postoperative beta-blocker therapy conveyed considerable protection against postoperative atrial fibrillation (p = 0.001) but was less effective in the older patients. Men were more prone to this arrhythmia (p = 0.02). Although these associations appeared significant, the logistic model proved to be a poor predictor of postoperative atrial fibrillation, which suggests that other factors not studied, or mere chance, may also be responsible.