We studied factor VIII related properties in 24 patients with increased platelet number. Twenty-one were affected by myeloproliferative disorders (eight had polycythaemia vera, 13 had essential thrombocythaemia) and three had secondary thrombocytosis. Normal levels of VIII:C and VIIIR:Ag were found while a significant (P less than 0.05) decrease of VIIIR:RCOF (43 +/- 13%) related to a lack of larger multimers of VWF (39 +/- 12%) was observed in 57% of patients with myeloproliferative disorders. A normal VWF pattern was found in the three patients with secondary thrombocytosis. The highest incidence of VWF abnormalities occurred in patients with essential thrombocythaemia (70%) in comparison with polycythaemic patients (38%). A significant (P less than 0.03) correlation between platelet count and the values of both VIIIR:RCOF and VWF multimeric pattern was observed only in patients with polycythaemia vera. The lowest levels of VIIIR:RCOF and the greatest loss of larger VWF multimers (less than 30%) were observed in two patients who presented bleeding symptoms at the time of study and a prolonged bleeding time. In addition, the relationship between VWF pattern and bleeding diathesis was supported by the fact that 75% of the patients with VWF abnormalities had bleeding history.