To examine the safety of using a high-flow three-way stopcock in a blood circuit during extracorporeal blood purification therapy, we studied the risk of development of hemolysis and clot formation in both ex vivo and in vivo extracorporeal circulation. In the ex vivo study, no significant difference was observed in the decrease in hematocrit (HCT) or increase in the potassium level between the three-way stopcock group and the control group. Nor was there a significant difference in the change in inlet pressure between the two groups. Thus, it was shown that the risk of hemolysis caused by the use of a high-flow three-way stopcock on both the outlet side and inlet side would be small. In the in vivo cases, there was no significant difference in the decrease in HCT or the increase in inlet pressure between the three-way stopcock group and control group, suggesting that it is clinically possible to incorporate a high-flow three-way stopcock into a blood circuit.