The aim of this study was to confirm or to reject whether plasma concentrations of individual fractions of middle molecular weight substances can become parameters for an adequate dialysis treatment and whether there exists a relation between that and some symptoms and signs in patients on regular dialysis. No changes in clinical condition, plasma concentration of middle molecules, or metabolic state after 5 years of short-time dialysis schedule were established. No difference was found in the plasma concentration of middle molecules after 6 months of continuous ambulatory peritoneal dialysis and in patients undergoing hemodialysis treatment. There was also no difference in glomerular filtration rate, blood urea concentration, dialysis index, and duration of regular dialysis treatment in patients with a middle molecular fraction 2 level lower and higher than 5.5 U/L. Elimination of fractions 2-4 and 6 by a 1-m2 Cuprophan coil dialyzer was similar to the elimination of creatinine. Only patients with acute respiratory infection had a fraction 2 higher than controls. Neither conventional parameters nor plasma concentrations of middle molecules can be indicators of an adequate dialysis. The well-being of patients and their metabolic state are the only criteria.