In heparin-free hemodialysis, the extracorporeal circuit is frequently flushed with normal saline. We report about an on-line hemodiafiltration (HDF) machine for performing heparin-free dialysis. The circuit is periodically flushed with fluid produced by filtration of the dialysis fluid. Because the fluid is derived from the volumetric dialysate circuit, it causes an equivalent ultrafiltration from the patient, and there is no need to allow for it in the fluid balance calculation. The use of the on-line HDF system in heparin-free dialysis is simple and cheaper than the saline-flush method. Automation of this system is possible and would result in even easier operation.