A prospective study of 50 consecutive patients undergoing cardiac valve replacement was designed to determine the incidence, types, and predisposing factors to postoperative arrhythmias. Patients were monitored continuously for the first 7 days following surgery.Thirty-seven patients (74%) experienced a total of 66 episodes of arrhythmia. Supra-ventricualr arrhythmias were the most common (43 of 66; 65%). The most frequent specific arrhythmia was atrial fibrillation (21 of 66; 32%). Arrhythmias occurred 77% of the time within the first 48 hours of surgery.Of 25 factors evaluated preoperatively in each patient, only two were found to predispose to postoperative arrhythmias. These were: (1) previous cardiac surgery; and (2) elevated blood urea nitrogen.There were four hospital deaths, representinig a hospital mortality of 8%. No deaths were due to a primary arrhythmia.It is concluded that whereas arrhythmias are a very common complication of cardiac valve replacement, early detection and treatment has lessened their significance as a cause of postoperative mortality and morbidity.