The laboratory features of acquired von Willebrand defect (AvWD) in myeloproliferative disorders (MPD) are characterized by a very high platelet count, a normal or prolonged bleeding time, a normal factor VIII and von Willebrand factor (vWF) antigen level, but a decreased vWF-ristocetine cofactor activity and collagen binding activity with a decrease or absence of large vWF-multimers simulating a type II von Willebrand factor deficiency. Although the pathogenesis of type II AvWD in MPD remains unclear, evidence is accumulating that the increased number of platelets in the circulating blood seems to be directly responsible for the observed decrease of large vWF-multimers in plasma leading to a spontaneous bleeding tendency at very high platelet counts. This observation may have clinical implications for the use of platelet anti-aggregants such as aspirin in MPD at platelet counts in excess of 1000 to 2000 x 10(9)/L.