Homeostatic imbalance may be an etiological factor in the development of acute coronary syndrome. Inherited resistance to activated protein C (APC) is a common disorder associat­ed with hype rcoagu I ability and lifelong risk of venous thrombosis. APC resis­tance is due to a single mutation in the gene coding for coagulation factor V (FV:Q506). The association of APC resistance with arterial thromboem-bolic disease, however, is still contro­versial. This study aimed to investi­gate the role of APC resistance in coronary artery thrombosis. We have studied the APC resistance {assessed by the ratio of the aPTT with and with­out added APC) in 66 adult patients under 50 years of age presenting with acute myocardial infarction. In addi­tion, plasma levels of anti-thrombin III (by coagulation method assay), pro­tein S (by radial immunodiffusion) and protein C activity (by coagulation method) were also determined. The results were compared with those of 16 apparently healthy individuals with matched age and sex without any thromboembolic events or bleeding tendency in their past history. APC re­sistance phenotype was considered positive when the APC sensitivity ratio was below or equal to the cut-off val­ue of 2.1. It was detected in one con­trol subject (6.25%) and in 12 patients with acute myocardial infarction (18.2%). The APC ratio was negative­ly correlated with LDH and AST (p values: <0.05 and <0.01 respectively) but not correlated with the lipid par­ameters or CK (total & MB). A step-wise multiple regression analysis re­vealed that LDH was the only