To elucidate the genesis and effect of fascicular block, 131 patients with acute Q-wave anteroseptal infarction were studied. Thirty-seven patients had new onset of fascicular block in the coronary care unit. The hospital mortality rate was 16%. Multivariate analysis was performed to determine the important variables related to the occurrence of fascicular block and hospital mortality based on 17 clinical variables obtained at the time of hospital admission. Serum potassium level and heart rate were the significant factors predicting the occurrence of fascicular block, whereas cardiac output and arterial oxygen tension were important for hospital mortality. Therefore, not only the rise in the heart rate but also the effect of the serum potassium level on the conduction system are independent factors related to the occurrence of fascicular block, and fascicular block in itself has no significant influence on the hospital mortality.