Indoxyl sulfate (IS) is a protein-bound solute, which is progressively retained in blood according to the decline of renal function. However, clinical relevance of excess IS in hemodialysis (HD) patients remains unknown. We measured serum IS and clinical parameters including pentosidine, carboxymethyllysine (CML) and homocysteine levels in 55 HD patients (age: 67 +/- 2 years, time on HD: 67 +/- 11 months, male/female = 30/25), and examined the relationship between IS and these data. Serum IS was markedly increased in HD patients (80.49 +/- 6.17 microg/ml) compared to normal range (< 1.9 microg/ml). IS was significantly and positively correlated with time on HD (p < 0.01), blood urea nitrogen (p < 0.01), beta2-microglobulin (p < 0.03), and protein catabolic rate (PCR) (p < 0.01). The patients with increased IS needed a higher erythropoietin dosage. Blood IS was positively correlated with pentosidine (r = 0.505, p < 0.01) and CML (r = 0.275, p < 0.05). In contrast, blood IS was not associated with nutritional and inflammatory parameters. A stepwise multiple regression analysis revealed that time on HD, PCR and pentosidine were significant determinants of IS. Serum IS was related to time on HD and dietary protein intake. Accumulated IS may be associated with enhanced carbonyl stress in chronic HD patients.