Quantitative assessment of infarct size is becoming increasingly important, particularly since this parameter appears to be a major determinant of prognosis. Morphologic, radioisotopic and electrophysiological techniques for assessing the extent of irreversible ischemic injury entail certain unavoidable limitations. Experimental and preliminary clinical findings suggest that estimation of the extent of infarction from biochemical markers in serum is practical and useful. Furthermore, prediction of infarct size during its early evolution may improve selection of patients for interventions designed to protect ischemic myocardium and may contribute to objective assessment of their efficacy.