Platelet function, blood coagulation and fibrinolysis were studied in 13 patients with myelofibrosis. The main abnormalities were impaired ADP-release and abnormal platelet aggregation with collagen. The delay period was prolonged to a degree varied from a few minutes to indefinitely. Platelet aggregation with ADP was impaired in 5 cases, while adrenaline-induced aggregation was abolished in 7 other patients. Spontaneous changes in the aggregation patterns were also observed. The bleeding time and clot retraction were normal. The platelet factor 3 availability was also normal, except in one case. Blood coagulation and fibrinolysis showed variable mild abnormalities. In one case a low-grade chronic disseminated intravascular coagulation was suspected, while a mild hypercoagulable state was present in other cases. The possible role of platelet function and of blood coagulation abnormalities in the interpretation of haemorrhage and thrombosis in myelofibrosis is discussed.