The AK200 ULTRA is a new device in the United States, but has been used for several years in Europe. The AK200 ULTRA is a single unit which is preprogrammed to provide standard hemodialysis as well as hemofiltration and hemodiafiltration with on-line preparation of the substitution fluid The AK200 ULTRA hemofiltration and hemodiafiltration is performed using a high flux membrane with high ultrafiltration rates compensated by volumetrically controlled infusion of on-line prepared substitution fluid. The AK200 ULTRA is, therefore, capable of providing dialysis treatments which can improve the removal of medium to high weight molecules without the difficulties associated with the manual measurement and administration of external substitution fluid.