Hypoalbuminemia predicts mortality in dialysis patients. It has been postulated that hypoalbuminemia in the dialysis population is a consequence of poor protein intake resulting from inadequate dialysis. To establish the cause of hypoalbuminemia in a group of 27 patients on peritoneal dialysis (PD), we determined the relationship between serum albumin concentration and a group of parameters including dialysis dose delivered (Kt/V), normalized protein catabolic rate (PCRn), transperitoneal and urinary albumin losses, and the serum concentration of two acute-phase proteins, C-reactive protein (CRP), and serum amyloid A (SAA). Serum albumin concentration could be predicted by a combination of transperitoneal albumin loss and either the serum concentration of CRP or of SAA. There was no relationship between weekly Kt/V or PCRn and serum albumin concentration. CRP and SAA significantly correlated with one another, but neither correlated with transperitoneal albumin losses. Hypoalbuminemia in PD patients is a consequence of transperitoneal albumin losses and of the acute phase response.