A natural relationship was revealed between the incidence of thrombotic complications and changes in the indicators of the blood coagulation system in patients with acute myocardial infarction. The totality of tests revealed two periods of maximum activation of the blood coagulation system, combined with inhibition of the anticoagulant system, on the 3-5th and 9-17th days of illness. These periods coincide with the time of the maximum incidence of thrombosis of the great vessels, disseminated intravascular coagulation and recurrence of myocardial infarction.