The electrocardiographic (ECG) patterns related to clinical spectrum and angiographic features were assessed in 41 patients with first myocardial infarction due to isolated left circumflex coronary artery (LCX) occlusion, and compared to those in 45 patients with right coronary artery (RCA)-related infarction. The occurrence of inferior Q waves was similar in patients with LCX and RCA occlusion, but lateral Q waves and an abnormal R wave in lead V1 were more frequently seen in patients with LCX-related infarction (46% vs 7% and 51% vs 4%, respectively). Compared with patients with LCX-related infarction without an abnormal R wave in lead V1 and those with RCA occlusion, patients with LCX-related infarction and an abnormal R wave in lead V1 associated with inferior and/or lateral Q waves had larger left ventricular end-diastolic and end-systolic volumes, lower ejection fraction, higher incidence of total occlusion of a dominant LCX without collaterals, and more cardiac events during follow-up. The study suggests that the presence of lateral Q waves and an abnormal R wave in lead V1 after myocardial infarction may be a useful marker of LCX occlusion, and that patients with LCX-related infarction may have different status of left ventricular function depending on the size of circulation and the status of residual flow to the infarct region during LCX occlusion.