The trend of some haemostatic parameters was investigated in a series of 186 myocardial infarction patients randomly allocated to sulphinpyrazone and placebo 15–25 days after the myocardial infarction episode in order to ascertain if one or more of these parameters may be considered as forecasting elements. The tests were performed at treatment allocation (basal values) and after 1, 6, and 12 months. In comparison with 44 healthy volunteers, the results have provided striking confirmation of ‘hyperactive’ platelets in the early phase of myocardial infarction expressed by the shorter bleeding time, increased plasmatic β- Mhromboglobulin, increased platelet factor 4 release and shorter heparin thrombin clotting time, and by the increased platelet sensitivity to threshold concentrations of adenosine diphosphate and collagen. Significant changes in bleeding time, platelet factor 4 release and heparin thrombin clotting time persist at successive testing times. Platelet aggregation by low collagen concentrations was inhibited in the sulphinpyrazone subsample patients.