Excess mortality and morbidity associated with right bundle branch and left anterior fascicular block were evaluated in 108 patients with block (age 74 +/- 10 years, 69% male) and 108 age- and sex-matched control patients with normal conduction. Clinical characteristics were similar initially except for more congestive heart failure in patients with block. Life table analysis revealed a higher 12 year mortality with block, even after omitting patients with moderate or severe congestive heart failure (risk ratio 1.47, p less than 0.05). Compared with control subjects, the group of patients with block had more sudden death and deaths of unknown cause, but a similar number of noncardiac and diagnosed cardiac deaths. More patients with block developed new second and third degree atrioventricular block or new overt coronary artery disease, but this finding did not support prophylactic pacing in asymptomatic patients. The importance of internal controls in assessing the natural history of clinical and electrocardiographic abnormalities is emphasized.