The purpose of this study was to examine factors associated with the development of complete left bundle-branch block and the prognosis in a group of people not in hospital, who had no clinical evidence of ischaemic or valvular heart disease. Twenty-nine cases of left bundle-branch block without clinical evidence of ischaemic heart disease were noted in the Manitoba cohort of 3983 men under observation since 1948. The most frequent electrocardiographic finding before development of left bundle-branch block was a normal electrocardiogram; left ventricular hypertrophy though infrequent, was the only abnormality significantly more common than in the rest of the group. The development of left bundle-branch block was associated with distinct leftward shift of the frontal plane mean QRS axis. The most frequent clinical cardiovascular event observed after development of the block was sudden death without previous clinical evidence of ischaemic heart disease. The five-year incidence of sudden death as the first manifestation of heart disease was 10 times greater in men with left bundle-branch block than in those without it.