1. The average and the range of blood recirculation during hemodialysis employing a coaxial counterflow single needle blood access catheter are markedly reduced compared to those observed in the same patients using a standard "Y" type needle and external flow direction control device. 2. In a large group of patients, the average measured recirculation of blood of 3.8% at a blood flow rate of 200 ml/min is sufficiently small as to have little or no effect on the efficiency of dialysis. 3. Recirculation increases with increasing extracorporeal blood flow rate but remains sufficiently low to not significantly affect increased dialysis efficiency obtained at higher blood flow rates. 4. The coaxial counterflow single needle catheter permits single fistula puncture, eliminates the need for a flow direction control device, and is associated with negligible blood recirculation. It offers a most attractive alternative to both double needle and present single needle blood access techniques for hemodialysis.