Eight hours of nightly tidal peritoneal dialysis (TPD) theoretically can provide uremia control equal that of continuous cyclic peritoneal dialysis (CCPD). To assess the in vivo validity of this prediction, six patients underwent mass transfer area coefficient (MTaC) measurements and dialysis using CCPD and TPD. CCPD consisted of five nighttime exchanges of 40 ml/kg and a daytime exchange of 20 ml/kg. TPD used an initial fill of 40 ml/kg and hourly tidal flows of 30 or 50 ml/kg. The nocturnal portion of CCPD lasted 9.7 hr (range 9.5–10 hr). TPD lasted 8.5 hr (range 8–9 hr) and was devoid of daytime dialysis. The patients consumed a diet containing 1.2 ± 0.07 g protein/kg body weight (range 0.7–1.7 g/kg) and had a pre dialysis blood urea nitrogen concentration of 52 mg/dl (range 18–82 mg/dl). The dialysate clearances of urea and creatinine were indexed to patient size and extrapolated to weekly values. CCPD provided a weekly creatinine clearance of 50 L/1.73 m2 and a Kt/Vurea of 2.06. TDP with an hourly dialysate flow of 30 ml/kg achieved a weekly creatinine clearance of 42.8 L/1.73 m2 and a Kt/Vurea of 1.73. When the hourly dialysate flow was increased to 50 ml/kg, these values improved to 53.3 L/1.73 m2 and 2.15, respectively. Dialysis efficiency equal to that of CCPD can be obtained using 8 hr of TPD when membrane characteristics (mass transfer area coefficient) and dialysate flow rates are appropriate. Patients with normal or above normal mass transfer area coefficients can obtain a weekly Kt/Vurea exceeding 2.0 using nightly high flow TPD.