Urea kinetic modelling (UKM) has been proposed as a tool for auditing the adequacy of CAPD and a total fractional daily urea cleared volume (Kt/V) of 0.25 suggested as the minimum adequate level. At the start of CAPD the kidneys contribute significantly to the total clearance and Kt/V often falls below 0.25 as renal function declines. We performed 3-monthly UKM measurements in 56 CAPD patients. These results were used to individualize exchange volume and frequency in an attempt to achieve a Kt/V > 0.25 and compensate for declining renal function in all patients over a study period of 1 year. The mean Kt/V was maintained over 0.29 over the study period. During this time the residual renal component of Kt/V fell significantly from 0.09 (SD +/- 0.07) to 0.06 +/- 0.08 (P < 0.001) while the dialysis component increased significantly from 0.20 +/- 0.05 to 0.24 +/- 0.05 (P < 0.005). This was achieved by increasing the mean daily exchange volume from 8.12 +/- 1.22 to 10.39 +/- 2.68 litres (P < 0.001). After a year, 15 patients had Kt/V < or = 0.25 despite maximum practical exchange volumes. Twelve patients dropped out of the study due to death (4), transplantation (2), and transfer to haemodialysis (6 patients, of whom 4 had frank uraemic toxicity). In most CAPD patients it is possible to compensate for declining renal function by increasing exchange volume, at least over 1 year. However, CAPD was unable to provide Kt/V > 0.25 in 40% of patients, despite individualization of the dialysis prescription.