BACKGROUND Identification of patients with myelofibrosis being at increased risk of acquired von Willebrand syndrome (avWS) would likely facilitate individualization of treatment and improve its outcomes. OBJECTIVES To determine the prevalence of avWS in patients with myelofibrosis, and to verify if individuals with and without this bleeding disorder differ in terms of their baseline clinical parameters. MATERIAL AND METHODS The study included 32 consecutive patients with myelofibrosis. avWS was diagnosed on the basis of abnormally low levels of von Willebrand factor and other routine tests. Patients with and without concomitant avWS were compared in terms of their demographic characteristics, present and past medical histories and laboratory parameters. RESULTS Concomitant avWS was found in 5 patients (15.6%). In 1/5 patients with avWS and in 8/27 persons without this bleeding disorder, myelofibrosis developed secondarily to polycythemia vera (n = 7) or essential thrombocytopenia (n = 2). As many as 4/5 individuals with avWS presented with clinical evidence of a bleeding disorder. The subjects with avWS differed from the remaining patients with myelofibrosis in terms of significantly lower activity of von Willebrand factor (vWF) and lower vWF to vWF antigen ratio. CONCLUSIONS All patients with myelofibrosis should be routinely evaluated for avWS with the panel of specific tests. Further, avWS should be the primary suspicion in each patient with myelofibrosis in whom clinical evidence of a bleeding disorder has emerged.