Precordial and posterior chest wall ST-mapping has been utilized in a patient with high anterolateral and true posterior myocardial infarction. In this patient evolution of the anterolateral component of the infarct was accurately delineated by changes in the the standard precordial leads. The true posterior component of the infarct was clearly diagnosed by detection of pathological Q waves and ST elevations in posterior chest wall maps, whereas right precordial standard leads had shown only suggestive reciprocal changes. The study illustrates the superiority of localized chest wall mapping over conventional 12-lead electrocardiography in detecting transmural infarction in high anterolateral and posterior regions of the left ventricle.1From the Cardiology Division, Thorndike Memorial Laboratory and the Department of Medicine, Boston City Hospital, and Boston University School of Medicine, Boston, Massachusetts 02118. Supported by Public Health Service Grants 5T01 HL 05986, HE 07299, HL 14646, and RR-533.