To elucidate indications for monitoring acute subendocardial infarcts, the clinical course of 15 consecutive patients with this diagnosis, who were admitted to a coronary intensive care unit, was reviewed. A history of hypertension or angina, or both conditions, was noted in 80%, and 13% had a myocardial infarction at some time prior to admission. Severe chest pain on admission was noted in 47%. In serial electrocardiograms, seven of 15 patients developed abnormal Q waves. Severe chest pain was found to be the only factor that could be correlated with development of transmural infarction, as it was present in six of seven patients developing Q waves, but only in one of eight patients who did not show progressive changes. Thus, no definite factors exist to differentiate those patients with acute subendocardial infarction who will proceed to transmural involvement.