The development of anti-heart mitochondrial antibodies (AHMA) after acute myocardial infarction (AMI) has been previously demonstrated in experimental studies. We performed a prospective study to check the incidence, variation curve and specificity of AHMA after AMI by using two different immunological tests: complement fixation and antihuman globulin consumption. Serial venous blood samples were drawn from the patients as follows: for the first 14 days after the event, once every other day; from 14 to 60 days after the acute event, once a week. The development of AHMA was demonstrated in the sera of 15/35 (45.4%) patients with AMI by complement fixation test using mitochondrial antigen obtained from normal human heart tissue compared to 12/33 (36.3%) by using mitochondrial antigen from the infarcted region. The antibodies exhibited cross-reaction with mitochondrial antigens obtained from human skeletal muscle, but did not react with heterologous heart mitochondrial antigens. According to the results of the antihuman globulin consumption test, the AHMA is immunoglobulin M (IgM). None of the patients developed postmyocardial infarction syndrome.