The relation between peak plasma-free-fatty-acid (F.F.A) concentration and size of infarct--as estimated by the plasma-creatine-kinase technique--was investigated in twenty patients with acute myocardial infarction. In eight patients with large infarcts (more than 65 g equivalents) mean infarct size was 136+/-21 g equivalents and mean peak F.F.A. value in the first 12 h was 1-99+/-0-14 mmol/l (mean +/-S.E.M.). In twelve patients with small infarcts, mean infarct size was 36+/-5 g equivalents and mean peak F.F.A. was 1-22+/-0-13 mmol/l (P less than 0-001). Experimental evidence suggests that high circulating F.F.A. concentrations could further extend ischaemic damage. The association between high plasma-F.F.A. and infarct size may give rise to a vicious circle which increases the severity of the ischaemic process in patients with high plasma-F.F.A.