OBJEVTIVE: To evaluate the peritoneal clearance of the middle molecule compared with that of the small molecule in incremental peritoneal dialysis(PD). METHODS: Peritoneal clearances of the creatinine and beta2-microgloblulin were compared in 57 continuous ambulatory PD patients with full dose 4 times exchange and in 54 incremental PD patients with 2 or 3 times exchange over 24 hours. The clearances were also compared when there were changes in the peritoneal dialysis regimen such as in the number of exchanges and dwelling time. RESULTS: Peritoneal creatinine clearance increased almost linearly along with the increase in the number of exchanges. In contrast, peritoneal clearance of beta2-microglobulin was 9.1+/-3.6 L/week, 8.8+/-4.4 L/ week, and 7.9+/-2.5 L/week respectively with 2, 3 and 4 exchanges per day, not different from each other. Peritoneal clearance of beta2-microglobulin did not change when there was an increase in the number of exchange from 2 to 3 times and 3 to 4 times over a period of 24 hours, whereas the peritoneal clearance of creatinine increased. Peritoneal clearance of beta2-microglobulin almost doubled from 5.4+/-2.7 L/ week with 2 times exchange over 12 hours per day, to 9.5+/-4.4 L/week with 2 times exchange over 24 hours, whereas the creatinine clearance did not change. CONCLUSION: In contrast to peritoneal clearance of small molecule which depends on the number of dialysate exchange, peritoneal clearance of middle molecule depends mainly on the total dwelling hours rather than the number of exchange per day in incremental PD. This can be another advantage of incremental PD since peritoneal clearance of middle molecules in incremental PD over 24 hours is comparable to that in full dose PD.