AS WOULD BE anticipated, disorders with moderately increased blood platelet levels are often characterized by thrombotic complications. Paradoxically, massive increases in the platelet count are more frequently associated with a hemorrhagic disorder.1 Thus, such diseases as polycythemia vera, chronic granulocytic leukemia, and essential thrombocythemia may present a hemophilia-like syndrome when the platelets exceed a million per cubic millimeter. The mechanism by which thrombocythemia leads to bleeding has been the subject of much speculation, but few of the suggested theories have been implemented by experimental verification. The present study concerns a case of thrombocythemia resulting from polycythemia vera. Data to be presented indicate that excess platelets display anticoagulant activity. Both thrombocythemic and normal platelets were found to be similarly anticoagulant at comparable concentrations. The findings suggest that the platelet anticoagulant effect may be at least partially responsible for the hemorrhagic manifestations of thrombocythemia. Report of Case A 71-year-old woman was