A group of well nourished young patients was examined during 2 longterm consecutive high efficiency dialytic schedules to detect possible clinical and biochemical differences. The patients were first studied during hemodialysis with a 1.2 m2 cuprophan membrane and blood flow of 400 ml/min and, then, during hemodiafiltration with an ultrafiltration rate greater than 100 ml/min by using high permeability membranes with a surface greater than or equal to 1.4 m2. As expected by the higher clearances obtained on hemodiafiltration, this technique resulted in a significant reduction of dialysis time and of BUN and serum creatinine, associated with a significant increase of hemoglobin, possibly related to a better deintoxication. Furthermore, the dry body weight decreased significantly during hemodialysis, whereas it increased significantly during hemodiafiltration, despite unchanged dietary habits, indicating possible differences in the nutrients utilization during the 2 dialytic schedules. Therefore, in chronically dialyzed patients, highly permeable synthetic membranes should be preferred to cuprophan because of a supposed better biocompatibility and should be used with the highest ultrafiltration rate in order to obtain a better dialytic efficiency and to avoid the risk of backfiltration.