Repeated investigation of 105 patients suffering from different blood disorders demonstrated a more frequent occurrence of disturbances in fibrinolysis (66% as in blood clotting tests (40%) indicating a compensated or decompensated intravascular blood clotting. In the group of patients with thrombocythaemia the disturbance of fibrinolysis as much as in 83% was present, and always in sense of insufficiency. In myeloproliferative diseases without proliferation of megakaryocytes the antithrombotic treatment improved both the fibrinolysis as well as the clotting disturbances. In thrombocythaemia the long-termed treatment with anti-platelet drugs, eventually with other antithrombotics favourably influenced the blood clotting parameters as well as the symptomatology from vascular occlusion, however the euglobulin lysis remained unchanged. In sense of the idea on the ineffective megakaryocytopoiesis in primary and other myeloproliferative diseases accompanying thrombocythaemia attention is called to the specificity of the fibrinolytic insufficiency in thrombocythaemia in comparison with other myeloproliferative diseases without thrombocythaemia.