Willebrand's factor is plasma protein involved in the processes of thrombo- and atherogenesis. Influencing the initial stages of platelet activation it may play a role in the pathogenesis of coronary heart disease (CHD). The content of Willebrand's factor was measured in 52 healthy subjects of different age and in 63 patients with CHD. Of these, 42 patients presented with acute myocardial infarction and 21 with angina pectoris. Use was made of a quantitative method based on ristomycin aggregation of formalin-fixed platelets from healthy subjects in the presence of Willebrand's factor of the test plasma. In angina pectoris patients, the highest content was detected if the disease took a graver course. In acute myocardial infarction, the content of Willebrand's factor was noted, in part of cases, to be high since the first hours of disease. In one of the cases the high content of the factor foretold the disease development. Myocardial infarction itself brought about another elevation of the factor content in plasma.