An optimal intraperitoneal volume (Vip) may be the most critical and least-used option to enhance higher clearances of small solutes. A higher Vip is more effective in increasing small solute clearances when compated to a lower Vip and more frequent exchanges. Additionally, a higher Vip generates minimal intraperitoneal pressure (Pip) in the supine position. Therefore, the highest tolerated Vip should be used at night while the child is sleeping in the supine position. Therefore, in 5 children, mean age 7 years, 4 months, on continuous cyclic peritoneal dialysis (CCPD) (Baxter HomeChoice), we tested the impact of Vip on tolerance (measured by Pip) measurements, and efficiency [assessed by daily Kt/Vurea and weekly creatinine clearance (Kcreatinine]. Basal Pip was determined for the usually prescribed Vip. Then Vip was increased during a morning outpatient study day, in 30-minute stages, stepping up to an increase of 25% of the basal Pip (upper limit 18 cm of water) level reached at the so-called optimized Vip used for the prescription of an optimal total dialysate volume for a 4-week period. The latest study week was compared to the prestudy week. The optimized Vip of 1230 +/- 70 mL/m2 was significantly higher than the basal Vip, 940 +/- 90 mL/m2. The dialysis dose improved both in terms of Kt/V urea from 0.23 +/- 0.02 to 0.29 +/- 0.03 and Kcreatinine from 60 +/- 5 to 76 +/- 9 L/1.73 m2/week. The supine position allowed the 25% Pip increment from 10.2 +/- 2.5 to 12.5 +/- 3.1 cm water, without any obvious clinical side effects.