OBJECTIVES: This study investigated the influence of the mechanical blood pump HeartMate II (HMII) (Thoratec Corporation, Pleasanton, California) on blood coagulation and platelet function. BACKGROUND: HMII is an implantable left ventricular assist device used for the treatment of heart failure. Patients treated with HMII have increased bleeding tendencies. METHODS: We measured agonist-induced platelet aggregation in 16 patients on HMII support. RESULTS: The von Willebrand factor (vWF)-dependent ristocetin-induced platelet aggregation was impaired in 11 of the 16 patients, of which 12 had experienced at least 1 minor or major bleeding episode. The impaired ristocetin-induced platelet aggregation was associated both with decreased specific activity of plasma vWF, presumably due to lack of high molecular weight vWF multimers, as well as with attenuated function of the platelets themselves. CONCLUSIONS: The results imply that HMII treatment is associated with impaired platelet aggregation, which may contribute to an increased tendency to bleed.