Adequacy of dialysis is addressed to minimize the negative consequences of underdialysis. The results of the NCSD demonstrated that urea kinetic modelling (UKM) is a useful tool in the measurement of adequacy of dialysis: Kt/V values of 1 are considered adequate to prevent complications when daily protein intake is appropriate. For CAPD no similar prospective studies exist, and consequently there is no information comparable to that for haemodialysis. Preliminary data have communicated controversial results about the usefulness of UKM for CAPD prescription. On the other hand creatinine kinetics has been proposed as an alternative to UKM to define adequacy of dialysis on CAPD. However, these results should be considered preliminary because of the small size of the series and the short observation period. The objective of this paper is to study the role of these two kinetic models in defining adequacy of dialysis in a long-term, selected CAPD population. We have studied 56 patients treated for at least 3 years on CAPD (3-11 years; mean observation period 5 years; 3996 patient-months). Urea kinetic model (UKM) (urea weekly (W) Kt/V and NPCR) and creatinine kinetics (Efficacy number (EN) and K) were determined annually. According to the present data we can conclude that Kt/V is an appropriate tool for defining adequacy in patients on CAPD. Values of weekly Kt/V greater than 1.7-1.8, with NPCR more than 1 g/kg/day, assure an adequate clinical outcome. Conversely, values on the Kt/V scale less than 1.7 entail a greater rate of complications. Our data also confirm that maintaining a serum albumin > 536-551 mumol/l (3.7-3.8 g/dl) is related to lower morbidity and mortality, and should be a major objective for CAPD patients. Consequently a relationship between dose of dialysis in terms of urea mass balance and protein nutrition can be established. After 5 years on CAPD, when all residual renal function is generally lost, Kt/V values in excess of 1.9 are difficult to achieve. Based on currently available information, we do agree with the interest of performing a prospective study on adequacy of dialysis based on the urea kinetic model in CAPD, which definitely defines its role. On the other hand, creatinine kinetics did not show sufficient discriminative capacity in terms of adequacy of dialysis. EN and Kt of creatinine values in the lower range (4.5-6 and less than 45-50 1/week respectively) were not necessarily associated with a poorer clinical outcome.(ABSTRACT TRUNCATED AT 400 WORDS)