SUMMARY To determine the relation between left anterior hemiblock and cardiovascular abnormalities in an ambulatory population, the clinical records and electrocardiograms of 16 600 male applicants for life insurance between 1966 and 1974 were reviewed. There were 413 applicants with left anterior hemiblock; they were compared with an age-matched control group. The subjects with left anterior hemiblock were divided into 2 subgroups based on the direction of the mean frontal place QRS axis: (a) greater than -30°but less than -60°, and (b) between -60°and -90°. The prevalence of left anterior hemiblock increased with age for each decade after the third. Left anterior hemiblock was not associated with cardiovascular abnormalities in 53-4 per cent of subjects age 30 and over and thus occurred as an isolated finding. There was no significant difference in the prevalence of isolated left anterior hemiblock when subjects with a mean QRS axis greater than -30°but less than -60°were compared with those with a mean QRS axis between -60°and -90°.When men of 30 and over with left anterior hemiblock were compared with an age and sex matched control group, there was a significant relation between electrocardiographic abnormalities, hypertension, and cardiac disease. There were no intergroup differences when subjects with a mean QRS axis greater than -30°but less than -600 were compared with subjects with a mean QRS axis between -600 and -90°. However, left anterior hemiblock was not a sensitive marker of clinical cardiac disease in these subjects since this diagnosis was absent in 86 per cent of subjects with left anterior hemiblock. In subjects under 30 a significant relation was present between clinical cardiac disease and left anterior hemiblock because of the high incidence of congenital heart disease in the left anterior hemiblock group.The significance of left anterior hemiblock in the general population remains controversial. Grant (1956) stressed the value of deviation of the mean manifest QRS axis in the frontal plane to the left beyond -30°as a sign of cardiac abnormality and postulated that left axis deviation signifies a conduction disturbance in the anterolateral wall of the left ventricle. On the basis of experimental evidence