Before the first 12 hours, diagnosis of early myocardial infarctions is always difficult for forensic pathologists. This study tested complement C9 expression in 121 formalin-fixed and paraffin-embedded heart samples by an immunohistochemical procedure. The heart specimens were separated into four groups: 33 cases in group 1 with typical ischemic damages histologically located; 20 cases in group 2 with death related to myocardial infarction on the basis of ischemic presentation on electrocardiogram but no obvious histological ischemic damage; 35 cases in group 3 with severe coronary disease without cause of death found at the autopsy; and 33 cases in group 4 without sign of myocardial infarction and without coronary disease. In the first group, all 33 heart samples showed a well-defined C9 expression in the necrotic areas. The second group, in 17 of 20 cases, showed positive areas for C9 expression. In the other three heart specimens, only a few stained cells were observed; whereas, the painful symptoms had begun less than 1 hour before death. The third group showed C9 immunopositive areas in 6 of 35 cases, few stained cells in 8 cases, and no C9 deposition in the 21 other cases. The last group showed no staining area. To avoid nonspecific C9 staining due to tissue autolysis, the researchers studied C9 expression during a controlled putrefactive process in four cases included in group 1; staining was found only in infarcted myocardial areas and was observed up to 10 days. Specificity of C9 expressions was evaluated to be 100 percent (89.4 to 100 percent) and sensitivity to be 85 percent (62.11 to 96.79 percent). Evaluation of immunohistochemical expression of C9 thus appears to be a highly sensitive and specific marker of early myocardial infarction, useful in forensic medicine if survival is more than 1 hour after the beginning of myocyte damage. 2 tables, 6 figures, and 27 references