To study whether there was an anti-cardiac myosin antibody (AMA) in serum of patients with myocardial infarction (AMI), relationship between AMA and the prognosis in patients with AMI was investigated. In 67 patients with acute AMI, AMA was assayed by ELISA and left ventricular structure and cardiac function were examined by echocardiography at the end of the first week after infarction and during a 6-month follow-up. The patients with AMI were divided into AMA-positive group and AMA- negative group. The parameters of left ventricular end-diastolic function and prognosis were compared between the two groups. Results showed that the AMA was positive in 18 patients with AMI, with a positive rate of 26.87%, while it was negative in 20 health donors. The locations of myocardial infarction in the two groups were similar. There were significant differences in Killip class I (22.22 % vs 55.10%, P＜0.05), decreasing of wall motion and ventricular aneurysm (92.85% vs 37.5%, P＜0.01) between the positive group and the negative group. During a 6-month follow-up, the mortality was higher in AMA positive group than in AMA negative group (38.89% vs 10.20%, P＜0.05). It is concluded that AMA can be detected in serum of patients with AMI and can serve as an important autoimmune marker. The autoimmune response might take place in AMI. AMA was associated with the left ventricular remodeling and the prognosis of AMI.