Maximum blood flow rates for arterial cannulae used in neonatal ECMO. The arterial cannulae used in neonatal ECMO cause hemolysis and red blood cell damage at elevated blood flows. Hemolysis in extracorporeal circuits has been found to occur with shear stress greater than 132 dynes/cm2, turbulence as measured by Reynold's number greater than 1,000, and velocity greater than 120 to 200 cm/sec. These parameters need to be considered when sizing the proper arterial cannula for a required flow rate. In-vitro measurements of the pressure drop across six arterial cannulae at varying flow rates were performed using human blood with a hematocrit of 43%. Shear stress, Reynold's number, velocity, and pressure drop were calculated for each catheter at flow rates from 50 to 1,000 cc/min. The maximum mean flow rate to maintain the shear stress, Reynold's number, velocity, and pressure drop within the accepted range, was determined for each cannula. Recommended maximum blood flow rates for each of the six cannulae are given. Internal diameter, length, and cannula geometry appear to be the factors most affecting the flow achievable without causing red blood cell damage and hemolysis. Ten French Biomedicus, 10 French Cook, and 10 French Elecath arterial cannulae appear best suited to deliver the range of blood flow rates used in neonatal ECMO. 