Abstract Mitochondrial and cytoplasmic isoenzymes of aspartate aminotransferase (AST) were studied in the sera of 42 patients following acute myocardial infarction and compared to creatine kinase (CK), lactate dehydrogenase (LDH) and alanine aminotransferase (ALT). Mitochondrial AST(ASTm) was detected in 93% (39/42) of patients. Maximum recorded ASTm activity was 59.5 ± 8.8 U/l and was found 39.4 ± 3.5 hours after the onset of symptoms (chest pain) of myocardial infarction. In contrast the maximum recorded cytoplasmic AST (ASTc) activity was greater (327 ± 23 U/l) and it occurred earlier (33.5 ± 2.2 hours) after onset of infarction compared to ASTm. ASTm correlated significantly ( p < 0.05) with ASTc, LDH and ALT but not with total CK or CK-MB. ASTc correlated significantly ( p < 0.05) with total CK, CK-MB and LDH but not ALT. Maximum recorded ASTm activity was significantly associated with the clinical assessment of left ventricular failure (Killip classification) but not with ventricular arrhythmias. In a subset of 15 patients evaluated with invasive hemodynamic measurements of cardiac output and pulmonary capillary wedge pressure. ASTm correlated significantly ( p < 0.05) and better than CK-MB with the hemodynamic assessment of left ventricular dysfunction. Thus ASTm can be readily identified in sera of patients after acute myocardial infarction and may be of value in the evaluation of patients with acute myocardial infarction.