Peritoneal dialysis (PD) patients lose significant quantities of protein and albumin during the dialysis procedure. The losses are greater in high transporters. The aim of the present study was to investigate the relationship between peritoneal membrane transport characteristics and protein losses. We studied 33 PD patients [14 men, 19 women; mean age: 53.5 years (range: 21-80 years)]. Fourteen patients had diabetes, and 22 were on automated PD. Dialysis adequacy was good, with a mean Kt/V of 2.63 (range: 1.51-4.89). Patients underwent a standard peritoneal equilibration test (PET). In addition, dialysate albumin (Alb) and protein (Pro) were measured at 0, 1, 2, and 4 hours, after lack of interference from unspent dialysate was ensured. Of the 33 patients, 23 were high or high-average transporters [based on 4-hour dialysate-to-plasma ratio of creatinine (D/PCr > or = 0.65)]. Protein losses owing to PD ranged from 3.5 g/day to 13.2 g/day (median: 5.9 g/day), of which 1.9-7.14 g/day (median: 3.21 g/day) was albumin. The 4-hour D/PCr correlated with the 4-hour D/PAlb (r = 0.62, p < 0.01), and 4-hour D/PPro (r = 0.63, p < 0.01). This finding persisted after correction for volume, indicating that it was not simply a concentration effect. The 4-hour D/PAlb and 4-hour D/PPro also correlated with the 24-hour PD albumin and protein losses. These results suggest a strong association between D/PCr and D/P for proteins. This observation is consistent with the increased protein losses through PD in high transporters and may be related to the inferior outcomes in this group.