To establish serum myosin light chain I (MLCI) as a severity and prognostic marker for patients with acute myocardial infarction (AMI), we measured the serum levels of MLCI in 71 patients with first AMI daily for 1 week after the onset and classified them into four groups by the peak LCI: group 1, ≥2.5 ng/ml but <10 ng/ml; group 2, ≥10 ng/ml but <25 ng/ml; group 3, ≥25 ng/ml but <50 ng/ml; and group 4, ≥50 ng/ml (MLCI grade). The patients in group 1 were likely to show non-Q-wave infarction. The patients in groups 1 and 2 were likely to show redistribution on exercise thallium-201 scintigraphy, suggesting frequent residual ischemia in these groups. The patients in group 4 were likely to show higher Forrester's subset and lower cardiac index at admission and lower left ventricular ejection fraction at discharge. Recurrent angina was equally found in all groups. Severe complications or death were found in patients in groups 3 and 4. Thus the MLCI grade can be used as a simple marker for evaluating the severity of patients with AMI. (Am Heart J 1998;135:329-34.)