Although hemodiafiltration is purported to provide better cardiovascular stability for dialysis patients; other possible benefits of this therapy have not been well defined. We have compared treatment with hemodialysis (HD) and hemodiafiltration (HDF) in 20 stable patients over a period of 18 months. Dialysis parameters (dialysate composition and flow, duration, dialyzer) were the same in the two periods except for the added convection of HDF and a higher tolerated blood flow in HDF. Cardiovascular parameters were remarkably similar in the two treatment periods, indicating that stable patients do not benefit further from this therapy in terms of these factors. The clearance of urea was significantly improved with HDF, which was reflected in a higher Kt/V and lower TACurea. We observed a significant correlation between Kt/V and PRU in both HD and HDF modes. This correlation was linear and the regression line was similar in both modes. The clearance of beta 2-microglobulin was also significantly improved by HDF compared to HD. Thus the benefit of HDF in stable dialysis patients is the improved clearance of small molecules and beta 2-microglobulin without increasing dialysis time. Further clinical benefits due to the improved clearance may only become apparent with longer follow-up.