Plasma thrombin-antithrombin III complex (TAT), FDP-D-dimer, activated protein C (APC)-protein C inhibitor (PCI) complex, and tissue type plasminogen activator (t-PA), PA inhibitor-1 (PAI-I) were significantly increased in patients with acute myocardial infarction (AMI) at onset. These patients exhibited a hypercoagulable state and protein C activation at onset. The plasma PCI level at onset of AMI was within the normal range, but was significantly decreased after percutaneous transluminal coronary angioplasty (PTCA). After PTCA, plasma t-PA, FDP-D-dimer, and plasmin-alpha 2-plasmin inhibitor were increased but APC-PCI complex and TAT were not. The decrease in PCI after PTCA may have been caused by the activation of fibrinolysis. PCI may play an important role in the inhibition of fibrinolysis in stimulated or damaged endothelial cells. These findings suggest that the protein C pathway plays an important role in the onset of AMI and after PTCA.