Rationale and Objectives: Sarcoidosis is a granulomatous disease of unknown etiology characterized by a Th1-mediated process. Previously we demonstrated a role for interleukin-18 in sarcoidosis. Here we examine the regulation of interleukin-18 in this condition. Methods: Cytokine levels were measured in sarcoid epithelial lining fluid using ELISA. We examined interleukin-18 promoter activity, mRNA and protein levels in response to epithelial lining fluid from individuals with either active or recovered sarcoid, protein purified derivative of Mycobacterium tuberculosis, beryllium-, zirconium- and aluminium-sulfate and lipopolysaccharide. Endotoxin levels were assessed in sarcoid, recovered and control epithelial lining fluid by Limulus Amebocyte Lysate Analysis. Allele-specific PCR was used to genotype 94 sarcoidosis patients and 97 controls for the interleukin-18 -607^A/C polymorphism. Species-specific PCR identified bacterial DNA in fluid samples. Results: Epithelial lining fluid from active sarcoids contained elevated levels of interleukin-18, interferon- and interleukin-12 compared to recovered patients and also contained significantly higher levels of endotoxin. Depletion of endotoxin from this epithelial lining fluid reduced its effect on the human interleukin-18 promoter in vitro. There was a higher frequency of the -607^C allele and -607^C/C genotype in the sarcoidosis population compared to controls, however this was not associated with a functional response to endotoxin treatment. Finally, bacterial 16s rRNA from Haemophilus influenzae and Moraxella catarrhalis was detected in sarcoid fluid samples. Conclusions: The pathogenesis of sarcoidosis is propagated through the actions of a Th1 driven response. This study shows that Gram-negative bacteria may contribute to this effect by up-regulating interleukin-18 expression.