The clinical implications of newly acquired left bundle-branch block (LBBB) were examined prospectively in the Framingham Study population. During 18 years of observation 55 people developed LBBB. The mean age at the onset of LBBB was 62; LBBB occurred largely in people with antecedent hypertension, cardiac enlargement, coronary heart disease, or a combination of these. Coincident with or subsequent to the onset of LBBB, 48% developed clinical coronary disease or congestive failure for the first time. Throughout the entire period of observation only 11% remained free of clinically apparent cardiovascular abnormalities. Within 10 years of the onset of LBBB, 50% had died from cardiovascular diseases. In men, the appearance of LBBB contributed independently to an increased risk of cardiovascular disease mortality. Comparison with age- and sex-matched control subjects free from LBBB confirmed that in the general adult population, newly acquired LBBB is most often a hallmark of advanced hypertensive or ischemic heart disease, or both.