Concerns regarding the impact of ultrafiltration failure on peritoneal dialysis and the effect of hypertonic glucose on the peritoneal membrane have lead to a search for alternative dialysates. Computer simulations based on the three-pore theory suggest that a combination of 1.36% glucose and 7.5% icodextrin (glucose polymer) offers an improved ultrafiltration profile. The aim of the present study was to investigate the ultrafiltration profile of this combination fluid. Prospective open study comparing 1.36% glucose, 3.86% glucose, 7.5% icodextrin, and the combination fluid (1.36% glucose/7.5% icodextrin). Sheffield Kidney Institute, Northern General Hospital, Sheffield, UK. 11 patients currently using peritoneal dialysis not previously exposed to icodextrin. Intraperitoneal volume was measured using a radioisotope dilution method. The combination fluid showed a biphasic ultrafiltration profile, with a steep initial increase in intraperitoneal volume, then a maintained plateau phase for the duration of the study dwell (7 hours). The final volume was greater than that with the 1.36% glucose dwell and the 7.5% icodextrin dwell. The fluid was well tolerated by the patients. These findings are in keeping with computer simulations using the three-pore model. The combination fluid offers an improved ultrafiltration profile, with a final volume similar to 3.86% glucose, while avoiding exposing the peritoneal membrane to high glucose concentrations. It may have a role as a long dwell to optimize ultrafiltration and possibly prolong peritoneal dialysis technique survival.