An early start of dialysis should improve survival and well-being, and reduce morbidity. But a rapid decline of residual renal function (RRF) is observed after the start of dialysis. Maintenance of RRF as long as possible is very important, because the kidneys do some things a dialysis membrane cannot do: tubular and endocrine function, larger clearances of middle molecules, some of them potentially toxic such as parathyroid hormone, granulocyte inhibitory protein, and substances inducing anorexia.