Bacterial infections of the residual dentin or infected pulp tissue are responsible for most cases of endodontic treatment failures. Persisting microorganisms in necrotic pulp tissue produce sulphur components such as methyl mercaptan and hydrogen sulfide as well as thioether derivatives. Although there is emerging evidence that these sulphur compounds stimulate immune cells and induce the inflammatory cascade, the immunological mechanisms of local and systemic inflammation have not been described. In this retrospective study we evaluated the ex-vivo immune response of peripheral blood mononuclear cells to sulphur compounds in 53 patients with clinical or radiologic endodontic treatment failure, 20 patients with clinical discomfort or radiological findings without previous endodontic treatment and a control group of 31 patients who had received successful endodontic treatment at least five years previously. Patients with endodontic abnormalities showed significantly higher ex-vivo sulphur compound-stimulated interferon-gamma (IFN-γ) and interleukin-10 (IL-10) levels as compared to the control group. The association between ex-vivo-stimulated cytokines and endodontically derived sulphur compounds was further substantiated by the fact that the number of IFN-γ and/or IL-10-positive patients decreased significantly 3-8 months after re-treatment of the root canal or tooth extraction. Furthermore, serum tumor necrosis factor-alpha (TNF-α) levels were higher in patients than in controls, and at the same time, the TNFA -308 G/A polymorphism was associated with endodontic treatment failure in our study population. We conclude that a cellular immune response to sulphur compounds contributes to the inflammatory process observed in relation to endodontic treatment failures.