To determine the incidence and prognostic significance of new postoperative conduction disturbances, 2,000 consecutive patients who underwent primary elective coronary bypass surgery were evaluated. One hundred eleven (5.5%) of the 2,000 patients developed a new intraventricular conduction defect that persisted to hospital discharge. Right bundle branch block occurred in 86 (85%), left bundle branch block in 5 (4%) and nonspecific intraventricular conduction defect in 9 (11%). One hundred of these 111 patients were successfully matched with others in the study population who had maintained normal intraventricular conduction during the operative period. Patients were matched on the basis of age, gender, absence of preoperative conduction disturbances, left ventricular function and bypass grafts to the same vessels. Follow-up of the two groups for a period of 1 to 76 months (mean 60 months) failed to show any difference in survival or cardiac events such as myocardial infarction, repeat coronary bypass surgery, coronary angioplasty and permanent pacemaker implantation. The appearance of right or left bundle branch block or a nonspecific intraventricular conduction defect after coronary bypass surgery does not appear to have an unfavorable impact on the long-term prognosis of these patients.