A method for measuring the peritoneal dialysis capacity (PDC) of the individual patient has been developed as an aid to treatment of patients with renal failure and peritoneal dialysis. The patient collects the data him or herself during an almost normal CAPD day using a carefully designed protocol whereby the nursing time is kept to a minimum. The three-pore model is used to describe the PDC with three physiological parameters: (1.) the 'Area' parameter (A0/delta X), which determines the diffusion of small solutes and the hydraulic conductance of the membrane (LpS); (2.) the final reabsorption rate of fluid from the abdominal cavity to blood (JVAR) when the glucose gradient has dissipated; and (3.) the large pore fluid flux (of plasma, JVL), which determines the loss of protein to the PD fluid. In the adult PD population (age 60, N = 97) the normal 'Area' parameter was 23,600 cm/1.73 m2, with an SEM of 650. The JVAR was 1.49 ml/min/1.73 m2 and JVL was 0.078 ml/min/1.73 m2. The PDC parameters were reproducible and could adequately predict the concentrations of the test solutes as well as that of beta 2-microglobulin. The results in terms of clearance, 'UF volume' and nutritional consequences were presented on easily understandable graphs, whereby patient compliance was improved. These physiological parameters are highly dynamic, as evidenced by the marked increases observed during peritonitis. It seems safe to conclude that PDC is a useful tool to achieve adequate dialysis and to enhance the understanding of PD exchange.