Alterations in various hemostatic factor have been identified as risk factors for survival after acute myocardial infarction (AMI). However, these clinical data are primarily limited to observations made during the postinfarct stage. We assessed the effects of 50 minutes of left anterior descending artery occlusion on several hemostatic factors and analyzed their changes with outcome in 18 Yorkshire swine. Blood samples were obtained from the systemic circulation at base-line and at 25 and 50 minutes of occlusion. Platelet aggregability and plasma antithrombin-III protein C, protein S, fibronectin, endothelin-1, as well as the metabolites of thromboxane and prostacyclin were measured. Of the 18 swine, 7 survived the infarct and 11 animals developed fatal ventricular fibrillation. Both groups demonstrated significant decreases in platelet aggregation, and a decline in plasma protein S when compared with baseline. Thromboxane, prostacyclin, and ondothelin-1 plasma concentrations were also markedly reduced at the end of occlusion. There were significant changes in antithrombin-III, protein C, and fibronectin levels between surviving animals and those that died of ventricular fibrillation. Acute coronary artery occlusion is associated with substantial changes in the hemostatic factors in swine. Plasma levels of fibronectin, antithrombin-111, and protein C differed between survivors and nonsurvivors and thus might serve as predictors of mortality due to fatal ventricular fibrillation during AMI. The mechanisms of these changes during the acute phase of AMI are unclear. Immediately AMI prognosis may be related to hemostatic changes not only after thrombolysis or spontaneous reperfusion, but also during the occlusion phase as well.