The purpose of this study was to determine the characteristics and incidence of abrupt occurrence of abnormal initial QRS forces that cannot be explained by acute myocardial infarction or left or right ventricular overload. Computerized data from 3175 patients with suspected acute infarction were reviewed to identify those in whom the ECGs revealed QRS complexes considered to be diagnostic (Q wave or markedly diminished R wave) in the presence of persistently normal profiles of both creatine kinase and lactic dehydrogenase isoenzymes. Lead misplacement had been minimized by obtaining multispace tracings and vectorcardiograms. Eight patients (0.25%) were identified. The abnormal forces were confined to leads V 1-3 in six, V 4-6 in one, and involved all precordial leads in the last. These QRS changes resolved completely within 6 days in all eight patients, which suggests that they did not have an acute infarction. This theory was supported by postmortem examination in one patient. An extremely low incidence (0.25%) has been documented for a syndrome characterized by transient loss of initial anterior forces with persistently normal isoenzyme profiles. Although no etiology could be determined, a transient conduction block of the septal fascicle of the left bundle could have been the cause in seven of the eight patients.