AbstractBecause the Silastic‐Teflon cannulas used for maintenance dialysis are far from ideal, usually failing in less than 1 yr, the factors affecting pressure drop in the cannula system have been evaluated theoretically and confirmed by actual measurements in vitro. Total pressure drop results from fluid friction, boundary layer separation and wall damping, and these effects are related directly to cannula size, shape, and material. By appropriate attention to these factors in cannula design, pressure drop in the system can be minimized. Soft Silastic tips for cannulas have been developed but show disappointingly slow flows and early clotting. These results are readily explained in terms of increased wall damping with soft materials. Commercially available hard Silastic cannula tips may be more satisfactory. Optimal cannula design requires large diameter cannula bodies, suitably shaped cannula tips, and the use of stiff tip materials. Such cannulas should allow the achievement of high blood flows with a minimum of boundary layer separation at the tip‐vessel junction and will be helpful in the prolongation of cannula life.