Platelet adhesion to endothelial cells, their aggregation and subsequent release of platelet-derived mediators are key steps in the pathogenesis of thrombosis and atherosclerosis. Using impedance technology the effect of inositol hexaphosphate (IP6) on platelet aggregation and adenosine triphosphate (ATP) release were simultaneously measured in whole blood obtained from healthy volunteers (n = 10). The platelets were activated with adenosine diphosphate (ADP) (10 microM), collagen (2 micrograms/mL), or thrombin (1 U/mL) in the presence or absence of IP6. IP6 significantly inhibited platelet aggregation induced with all agonists in a dose-response manner (p < 0.0001 for ADP and collagen, p = 0.0103 for thrombin), with the IC50 values of 0.9, 1.6 and 0.8 mM. Secretion of platelet dense granule content was measured in parallel. IP6 strongly and significantly reduced agonist-induced ATP release (p = 0.00247 for ADP; p = 0.0074 for collagen; p = 0.0069 for thrombin). These data demonstrate that IP6 effectively inhibits human platelet aggregation in vitro, suggesting its potential in reducing the risk for cardiovascular disease.