In our study, the tryptophan degradation via the kynurenine pathway and neopterin levels as markers of cellular immune activity were evaluated in hemodialysis (HD) patients in order to confirm the data of previous studies. In this case-control study, serum tryptophan and kynurenine concentrations were determined by HPLC, neopterin levels were measured with ELISA in 54 hemodialysis patients and 33 healthy controls. There were significantly increased kynurenine, kynurenine/tryptophan ratio and neopterin levels, and decreased tryptophan levels in hemodialysis patients compared with the healthy controls (p=0.0001). In our study, tryptophan degradation and neopterin levels are increased in HD patients. These changes may result from impaired excretion of the compounds or neopterin production and tryptophan degradation in the course of inflammation and immune activation during hemodialysis.