To detect right ventricular involvement, lead V4R was recorded within 10 hours of the onset of chest pain in 42 consecutive patients admitted with acute inferior wall myocardial infarction. One week after the acute infarction, multigated equilibrium radionuclide ventriculography was performed to assess right and left ventricular ejection fraction. Two weeks after the acute infarction, coronary angiography was performed to determine the site and location of the obstruction leading to the infarction. Seventeen patients had an obstruction in the right coronary artery proximal to the first branch to the right ventricular free wall (group 1); all of these had ST segment elevation in lead V4R. Fourteen patients had an obstruction in the right coronary artery distal to the first branch to the right ventricular free wall (group 2); only two of these patients had ST segment elevation in lead V4R. In 11 patients, the obstruction was located in the circumflex coronary artery (group 3); none of these had ST segment elevation in lead V4R. Nineteen patients had ST segment elevation of 1 mm or greater in lead V4R (group 4). Left ventricular ejection fraction was not different among the four groups of patients, although the right ventricular ejection fraction was significantly lower in group 1 and group 4 patients. It is concluded that ST segment elevation in lead V4R reliably identifies the group of patients with inferior wall myocardial infarction with depressed right ventricular function. This phenomenon persists for at least 1 week after infarction.