In 322 patients undergoing isolated coronary artery bypass grafting, the possible factors responsible for the development of intraventricular conduction disturbances were investigated. In 18 patients (5.6%), complete right bundle branch was observed perioperatively, 2 of whom also demonstrated left anterior hemiblock. Left bundle branch block, either complete or incomplete, was not observed. Patients demonstrating perioperative complete right bundle branch block were further characterized by the finding that 14 of 18 (77.7%) had preoperative inferior wall infarction as opposed to only 34% in the remaining 304 patients. Fifteen of the 18 patients (83.3%) had prolonged aortic cross-clamp times in contrast to only 37.5% of the remaining patients. Three-vessel disease, present in 16 of the 18 (88.9%) patients, was less frequently present (56.2%) in those in whom complete right bundle branch block did not develop perioperatively. Application of the chi 2-test showed significant differences in all of the latter variables. The perioperative onset of complete right bundle branch block may be due to several factors. The results of this study indicate that the extent of scar tissue and arteriosclerotic changes as well as the ischemic time during surgery may play a decisive role. In ten of the patients, the perioperatively-incurred complete right bundle branch block was irreversible. Postoperative angiographic studies revealed no relationship between block development and graft occlusion.