The purpose of this study was to determine the long-term outcome of patients with bundle branch block (BBB) who have no clinical evidence of cardiovascular disease. Among 110,000 participants in a screening program, 310 subjects with BBB without apparent or suspected heart disease were identified. Their out-come after a mean follow-up of 9.5 years was compared with that of 310 similarly screened age- and sex-matched controls. Among the screened population, isolated right BBB was more prevalent than isolated left BBB (0.18% vs 0.1%, respectively; p < 0.001), and the prevalence of each abnormality increased with age (p < 0.001). Total actuarial survival was no different for those with left BBB or right BBB and their respective controls. Cardiac mortality, however, was increased in the left BBB group when compared with their controls (p = 0.01, log rank test). Left BBB, but not right BBB, was associated with an increased prevalence of cardiovascular disease at follow-up (21% vs 11%; p = 0.04). In the absence of clinically overt cardiac disease, the presence of left BBB or right BBB is not associated with increased overall mortality. Isolated left BBB is associated with an increased risk of developing overt cardiovascular disease and increased cardiac mortality.