The interrelationship between Kt/V (urea), protein catabolic rate, dialysis membrane, nutritional status and outcome: These correlations are summarized in Figure 3. A given dialysis membrane will allow a given Kt/V (urea) and hence, influence urea removal. The balance between urea removal and urea generation will determine the urea pool and the urea concentration for a given volume of distribution. It is postulated that this urea concentration in some way causes 'biofeedback' determining the protein intake of a given patient which, of course, will influence the protein catabolic rate and hence, the urea generation. The protein catabolic rate may be adversely influenced by the use of bioincompatible membranes which produce activated complement, IL-1, tissue necrosing factor, etc. In addition, the membrane will determine the Kt/V (other toxins; 'middle molecules') which may also influence protein intake. The protein intake will affect the overall nutritional status which, in turn, has a major impact on morbidity and mortality. We suggest that the most effective way to influence morbidity and mortality is to ensure that a Kt/V (urea) well in excess of 1 (perhaps between 1.4 and 1.6) is delivered to the patient if a cellulosic membrane is used. It may be possible to deliver less than this value using a biocompatible membrane which has increased Kt/V (middle molecules) for a given Kt/V (urea). Whatever the method of dialysis or Kt/V (urea) applied, attention should also be gi en to nutritional assessment of the patient, and amongst the tools available to the clinician is the estimation of the protein catabolic rate by urea kinetic modelling.(ABSTRACT TRUNCATED AT 250 WORDS)