BACKGROUND: Acquired immunity is impaired in hemodialysis (HD) patients. Indoleamine 2,3-dioxygenase (IDO) is inducible by inflammation and through tryptophan depletion and generation of kynurenine pathway products suppresses adaptive immune response. In the present study plasma IDO levels were assessed in HD patients. Its effect on response to HBV vaccination program was evaluated.PATIENTS AND METHODS: Sixty-six HD patients and twenty-four healthy volunteers enrolled into the study. All the HD patients were initially vaccinated with four double doses of recombinant HBV vaccine. All doses were repeated in patients who had not responded after complete first vaccination series. Only one boost dose was being administered in patients with initial adequate antibody levels against the HBV surface antigen (anti-HBs>10I U/L) who then presented with reduced anti-HBs levels. IDO, CRP, IL-6 and TNF-α were measured by means of ELISA.RESULTS: Compared to healthy volunteers, IDO levels were twice higher in HD patients. CRP, IL-6 and TNF-α were also much higher in HD patients. IDO levels were almost twice higher in HD patients with inadequate response to HBV vaccination, than in those with adequate immune response. CRP, IL-6 and TNF-α did not differ between the two patients' groups. IDO was negatively correlated with all markers of inflammation in HD patients.CONCLUSION: IDO is increased in HD patients. It is possible that after its initial upregulation due to chronic inflammation, IDO curtails its own provoking agent, i.e., inflammation. Increased IDO suppresses adaptive immunity in HD patients, as it is assessed by the response to HBV vaccination.