Cardiovascular abnormalities were identified prospectively in all 70 persons who developed complete right bundle-branch block (RBBB) in The Framingham Study during 18 years of biennial follow-up. Most were hypertensive before the appearance of RBBB. Although the initial appearance of RBBB was usually unaccompanied by overt clinical events, the subsequent incidence of coronary disease and congestive failure was two and one-half and four times greater, respectively, than that in matched control subjects without RBBB. The incidence of cardiovascular disease mortality was almost three times greater in persons who developed RBBB than in an age-matched sample of the population-at-large. This excess of cardiovascular disease mortality was related primarily to the high prevalence of associated cardiovascular abnormalities. Only 21% remained free from clinically apparent cardiovascular abnormalities. A QRS duration of greater than or equal to 130 ms and a QRS axis between -45 degrees and -90 degrees identified those most likely to have associated cardiovascular abnormalities.