The short- and long-term prognosis of acute intraventricular (iv) conduction defects is analyzed in 59 patients admitted to the coronary care unit with acute myocardial infarction. In-hosptial mortality of patients with iv conduction disturbances was more than twice (30%) the mortality of patients without iv conduction defects (13%; p less than 0.001). Mortality was very high among patients with all forms of incomplete trifascicular block or complete right bundle branch block. Among survivors of the group with conduction defects, the late death rate was significantly higher than in survivors of the group without conduction disturbances (25% and 8% respectively: p less than 0.001). The short- and long-term prognosis of acute conduction defects in myocardial infarction depends on the extent of necrosis. Permanent endocardial pacing is therefore indicated only among patients whose death risk is related to conduction disturbances.