Over a 3-year period of observation, the electrocardiogram in an adult patient with an ostium primum atrial septal defect changed from first-degree atrioventricular block alone to right bundle-branch block and left-axis deviation (left anterior hemiblock) with variable atrioventricular block. Coronary atherosclerosis was excluded by selective coronary arteriography. Although right bundle-branch block and left anterior hemiblock are commonly present at birth in patients with endocardial cushion defect, this is believed to represent the first reported instance in which these electrocardiographic abnormalities have developed under observation in later life. It is postulated that chronic hemodynamic stress contributed to the progression of disease in the conduction system.