Based upon earlier findings of our group that not only the synthesis of antiaggregatory prostaglandins by the vessel wall is altered in atherosclerotic vascular tissue of patients, but also the sensitivity of their platelets to these prostaglandins is changed, we examined a well defined group of 50 patients with myocardial infarction before the age of 40 years. In all patients the coronary heart disease was verified by angiography. The sensitivity of the platelets was examined in relation ot the smoking habit, the type and presence of hyperlipoproteinemia, one, two and three vessel disease and familial history. Except for hyperlipoproteinemia and smoking, all the patients were free of any additional risk factor. It is demonstrated that the platelet sensitivity to the antiaggregatory prostaglandins is significantly diminished in hyperlipoproteinemics in comparison to a control group.