The accurate estimation of myocardial damage is desirable for the assessment of myocardial protection and surgical treatments. The purpose of the study was to estimate myocardial damage by measuring cardiac myosin light chain I (CMLCI). Forty-nine patients undergoing cardiac surgery (for angina or valvular disease) were arbitrarily divided into four groups according to the maximum CMLCI level. Group 1: max CMLCI < 10 micrograms/L (37%); Group 2: max CMLCI 10-20 micrograms/L (39%); Group 3: max CMLCI 20-30 micrograms/L (16%); Group 4: max CMLCI > 30 micrograms/L (8%). Electrocardiogram (ECG) and serum creatine kinase MB isoenzyme (CK-MB) were conventionally used as standards of myocardial damage, and compared with CMLCI. Perioperative myocardial infarction, injury and minimum damage were determined by combinations of ECG pattern changes and CK-MB levels. The max CMLCI level was usually seen on the third postoperative day. None of the patients in group 1 had any ECG changes. The number of patients with ECG changes was much higher as the max CMLCI level increased, and evidently increased when the max CMLCI was over 20 micrograms/L. The number of patients with high CK-MB > 100 micrograms/L followed the same pattern. Furthermore, perioperative infarction was only seen when the max CMLCI was > 30 micrograms/L. The peak CMLCI level was significantly higher in the infarction group than injury and minimum damage groups. This study showed that CMLCI was able to estimate the actual extent and severity of the myocardial damage and enhanced the diagnosis of perioperative infarction.