Results obtained by a simple in vitro clot lysis test in polycythemia vera, polyglobulia, essential thrombocythemia and chronic immun thrombocytopenic purpura are reported. Increased lysis resistance of the platelet-rich clots was demonstrated in polycythemia and essential thrombocythemia, ostensibly caused by the high plasma level of plasminogen activator inhibitor-1. Following retraction of platelet-rich clots no further increase of lysis-resistance occurred in polycythemia and essential thrombocythemia. This phenomenon is most likely due to abnormal platelet function. The well known thromboembolic complications may at least partly result from the impaired fibrinolysis in diseases with high platelet counts.