Plasma and platelet von Willebrand factor (vWF) measurements, multimeric pattern and subunit composition of plasma vWF were obtained in 29 consecutive patients with chronic myeloproliferative syndromes. In the 8 patients with chronic myelogenous leukemia (CML), plasma vWF was significantly higher than in the 11 patients with essential thrombocythemia (ET) and in the 10 patients with polycythemia vera (PV). The RiCof/vWF:Ag ratio was low in all these groups of patients (mean 0.64 +/- 0.1, 0.66 +/- 0.2, and 0.61 +/- 0.2; normal 0.97 +/- 0.2). Bleeding time was prolonged (> 7.5 min) in 1/8 CML patients, 1/10 with PV, and 3/11 with ET. Plasma vWF multimers showed a minor loss of the largest multimers in 3/8 patients with CML, 4/10 with PV, and a more severe reduction in 9/11 ET patients. The latter pattern correlated with an abnormal proteolysis of vWF, expressed by a major increase of the 140-Kd fragment and decrease of the intact 225-Kd subunit in ET patients, whereas the 176-Kd fragment was significantly increased in all the subgroups of patients. Platelet vWF was significantly higher in CML patients in comparison to ET and normal controls. However, minor losses of the larger multimers were evident in all the subsets of patients. In ET patients also the intermediate forms were lacking in platelets, accompanied by a significant decrease of platelet RiCof. This abnormality was significantly correlated with the occurrence of bleeding symptoms in PV and ET patients (P = 0.007; Fisher's exact test). In conclusion, plasma and platelet vWF abnormalities are common findings in myeloproliferative syndromes and are more severe in ET. The more pronounced platelet vWF abnormalities in ET may reflect the more frequent bleeding symptoms observed in this disorder.