Increasing numbers of patients receive peritoneal dialysis using noncontinuous methods such as nightly intermittent peritoneal dialysis (NIPD) rather than continuous ambulatory peritoneal dialysis (CAPD). We hypothesized that blood solute levels before and after NIPD would be large enough to produce significant variability in formulas based on the continuous peritoneal dialysis (PD) model. We found no diurnal differences in serum creatinine in NIPD or CAPD. However, our data demonstrate a 7.9% difference in serum urea measurements from evening to morning in patients treated by NIPD. This mathematical difference contributes to a 6.3% difference in the calculated value of KT/V and a 9.4% difference in the calculation of total urea clearance (liters per week) in these patients. By contrast, no difference in serum values or in calculated values of adequacy could be shown in patients on CAPD. These observations support the premise that CAPD represents a steady-state condition. NIPD patients demonstrate variability in serum levels of urea which may result in inaccurate calculations of dialysis adequacy. When blood samples are obtained in the morning soon after completing a cycle of NIPD, dialysis adequacy as measured by KT/V or total urea clearance (but not by total creatinine clearance) may be systematically overestimated.