The ECG was studied in 100 patients with coarctation of the aorta. Thirty-nine of these had additional cardiac lesions, usually aortic valve disease. Evidence of left ventricular hypertrophy was commoner in the presence of a complicating lesion. Among those with lone coarctation left ventricular hypertrophy was recognised in 52% of those below 25 but was present in 89% of the older patients. Among the younger patients there was good correlation between the presence of ECG abnormalities and cardiac enlargement on X-ray; in the older group the changes of left ventricular hypertrophy were usually found irrespective of heart size. Partial or complete right bundle branch block was present in 18% of those with isolated coarctation, mainly among the younger patients, and in 8% of those with additional defects. It is suggested that the development of left ventricular hypertrophy gradually obscures the bundle branch block pattern which is common in young patients with lone coarctation.