The changes in blood coagulative properties, which occur in the treatment with medical leeches, were studied in 57 patients with coronary heart, rheumatic and chronic pulmonary diseases. Hirudin therapy was established to affect blood coagulation. The leech salina penetrated into the skin wound to block hemostasis. Having entered the circulating blood, some part of the saliva normalized its impaired coagulative properties. The correction of the above abnormalities and related bleeding indicate it beneficial to use hirudin therapy in secondary coagulopathies and circulatory decompensation.