Whether or not a significant interrelation exists between patterns of the body surface isopotential map and sites of coronary obstruction and left ventricular asynergy in old myocardial infarctions was investigated. The QRS departure map (Dmap) was defined as the area of which the potentials were lower than the mean-2 SD of normal controls. A Dmap developed on the upper area above the 5th intercostal space between the vertical lines of lead V 2 and lead V 3 in the anterior myocardial infarction, and on the lower parts of the right anterior chest and the left back in the inferior myocardial infarction. The asynergy of seg. 1 manifested by left ventriculograms corresponded to the Dmap of the upper points of the left anterior axillar line, and seg. 2 corresponded to the upper area above the 5th intercostal space between leads V 2 and V 3. The Dmaps were observed in the area of two intercostal spaces lower than usual V 2 and V 3 points between leads V 2 and V 3, and in the lower point of the left posterior axillar line corresponded to asynergy of seg. 3 and seg. 4, respectively. Asynergy of seg. 5 was represented in the leads of the lower part of the right anterior chest and the back. In patients with left anterior descending coronary artery (LAD) lesions which involved the 1 st diagonal branch, the Dmap shifted significantly to the left and upper area in comparison with the Dmap of patients with a LAD obstruction distal to the branch. Patients with a LAD lesion involving the 1 st diagonal branch showed a Dmap in the area at one costal space higher between the vertical lines of leads V 1 and V 2 than patients with an intact diagonal branch. The Dmap could separate the inferior infarction due to right coronary artery (RCA) obstructions from the infarction due to left circumflex coronary artery (LCX) lesions: a RCA lesion showed a Dmap on the lower part of the right anterior chest, and a LCX lesion showed a Dmap on the lower part of the left back. These data suggested that the Dmap is of use in locating myocardial and coronary artery lesions in old myocardial infarctions.