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ST segment depression in leads V2 to V4 in a clinical and biochemical context of myocardial infarction is usually interpreted as a sign of non-Q wave anterior walls infarction. In order to determine if this clinical electric entity could indicate transmural posterior or posterolateral infarction, as recently suggested, we undertook a prospective study of 328 primary myocardial infarctions. Isolated ST depression in leads V2 to V4 was observed in 28 patients (8.5%). It was maximal in V3 (1.8 +/- 0.7 mm) or V4 (2 +/- 1 mm). The T wave was always positive. All these case had segmental wall motion abnormalities of the left ventricular posterolateral wall on 2D echocardiography. The Q wave confirming the transmural character of the infarct was observed in leads V7, V8 and V9 on average 33 hours after the onset of pain (10-56 hours) as did the increase in the R/S ratio in leads V1 and V2. Coronary angiography performed in 26 patients showed significant disease of the left circumflex artery in all cases. This was isolated (39%) or associated with left anterior descending (15%), right coronary artery disease (19%) or both (27%). In conclusion, isolated ST segment depression in leads V2-V4 in the clinical context of acute myocardial infarction indicates a transmural posterior localisation of the necrosis. It corresponds to reciprocal subepicardial posterior ischaemia. In cases of inferior infarction, it reflects postero-lateral extension rather than associated anterior wall ischemia.