The value of the accelerated idioventricular rhythm (AIVR) as a marker for myocardial necrosis and/or reperfusion was prospectively studied in 87 patients admitted with persistent ischemic chest pain. All patients received streptokinase. Necrosis was diagnosed by new Q waves and an increase in plasma enzymes. Reperfusion was documented angiographically. Myocardial necrosis occurred in 72 patients and reperfusion in 70 patients, 58 of whom had myocardial necrosis. Of 27 patients with AIVR, 26 had both necrosis and reperfusion (p less than 0.001). AIVR started after a long coupling interval to the preceding sinus rhythm and was regular. Configuration depended on the reperfused infarct vessel. Reperfusion of the left anterior descending branch showed most configurations of AIVR and with the least QRS width. Reperfusion of the circumflex branch never had a left bundle branch block-like configuration. AIVR from reperfusion of the right coronary artery never had an inferior axis. AIVR occurring during persistent ischemic chest pain is a marker for both myocardial necrosis and reperfusion of the infarct vessel. AIVR starts with a long coupling interval and is regular. The QRS configuration may be useful for the noninvasive identification of the infarct vessel.