Nafamostat mesilate (NM), a synthetic protease inhibitor, is the most commonly used anticoagulant in the setting of extracorporeal circulation (ECC) in patients with bleeding tendency. It inhibits both platelet aggregation and activation of coagulation factors. Although it has been reported that NM disaggregates aggregated platelets, little is known about such an effect in the setting of hemodialysis therapy (HD). We examined the effects of NM on adenosine 5′-diphosphate (ADP)-induced platelet aggregation and disaggregation using platelet-rich plasma obtained from 6 HD patients. The platelet aggregation was stimulated by 3μM ADP and change of aggregation was monitored by an aggregometer. NM adjusted to the final concentrations of 0.1 (1.9 × 10–7), 1.0 (1.9 × 10–6), 10, (1.9 × 10–5), and 100 (1.9 × 10-4) μg/ml (M) or veronal-buffered saline (VBS) as control was added before or after to the stimulation of ADP. NM not only inhibited platelet aggregation, but also disaggregated already aggregated platelets at concentrations of 1.0 μg/mln or higher. Moreover, NM almost completely disaggregated at 100 μg/ml. This NM concentration of 1.0 μg/ml was lower than the therapeutic concentration in ECC of HD (i.e., 10–5M). Both inhibitory and disaggregatory effects of NM expressed a dose-related dependency. Our results suggest that NM can exert both aggregation inhibitory and disaggregatory effects on platelets of HD patients within the therapeutic concentration.