Two hundred consecutive patients undergoing only coronary bypass surgery were studied. Forty-five patients (group A) developed new fascicular conduction blocks and 155 patients (group B) did not. The 45 patients in group A developed the following fascicular conduction blocks: right bundle branch block 47%, right bundle branch block and left anterior hemiblock 8%, right bundle branch block and first-degree atrioventricular block 2%, left anterior hemiblock 11%, left bundle branch block 18%, right bundle branch block-left anterior hemiblock and first-degree atrioventricular block 5%. There were no significant differences in sex, incidence of diabetes, number of grafts performed, ejection fraction (less than 55%), and perioperative infarction. Group A patients were older (p less than 0.01). Hypertension was found frequently in group A (27 vs 45 patients; p less than 0.01) and was present for a mean of 12.4 years in group A and 4.9 years in group B (p less than 0.01). Preoperative use of digitalis was found in 14 (31%) patients in group A and in 18 (12%) patients in group B (p less than 0.01). Twenty-one (47%) patients in group A had significant disease (greater than 70%) of the left main coronary artery as compared to 17 (10.9%) in group B (p less than 0.001). There was no difference in the recurrence of angina or the survival rate at 14 months. In conclusion, the incidence of new fascicular conduction block after bypass surgery is 22.5%. Long-standing hypertension, left main coronary disease, and the preoperative use of digitalis appear to be predisposing factors. New fascicular conduction block does not affect prognosis.