The change of mitochondria aspartate aminotransferase (m-AST)/soluble-AST (s-AST) ratio was examined in 22 cases of acute myocardial infarction (AMI). The m-AST/s-AST was 40.8 +/- 18.9% at admission to a hospital (2.9 +/- 1.6h). The m-AST/s-AST decreased to normal value rapidly after peak and then increased again gradually. The decrease ratio of m-AST/s-AST per minute at early stage of 8 cases, who were succeeded to reperfusion, was 0.28 +/- 0.20%, and that was significantly higher than of conventionally treated 7 cases and non-reperfused 7 cases (0.11 +/- 0.07%). These results indicated that (1) m-AST/s-AST may be an excellent indicator for AMI in early stage. (2) The decrease ratio of m-AST/s-AST would predict whether reperfusion is successful or not at an earlier stage of AMI.