Rheumatoid arthritis (RA) is a chronic inflammatory disease in which fibroblast-like synoviocytes (FLS) exhibit an aggressive phenotype. Although the mechanisms responsible are not well defined, epigenetic determinants such as DNA methylation might contribute. DNA methyltransferases (DNMTs) are critical enzymes that establish and maintain DNA methylation. We evaluated whether proinflammatory cytokines might contribute to differential DNA methylation previously described in RA FLS through altered DNMT expression. FLS were obtained from RA and osteoarthritis (OA) synovium at the time of total joint replacement. Gene expression was determined by quantitative real-time PCR and protein expression by Western blot analysis. DNMT activity was measured with a functional assay, and global methylation was determined by an immunoassay that detects methylcytosine. Resting expression of DNMT1, -3a, and -3b mRNA were similar in RA and OA FLS. Western blot showed abundant DNMT1 and DNMT3a protein. Exposure to IL-1 decreased DNMT1 and DNMT3a mRNA expression in FLS. Dose responses demonstrated decreased DNMT expression at concentrations as low as 1 pg/ml of IL-1. DNMT mRNA levels decreased rapidly, with significant suppression after 2-8 h of IL-1 stimulation. IL-1 stimulation of OA FLS did not affect methylation of LINE1 sites but led to demethylation of a CHI3L1 locus that is hypomethylated in RA FLS. Chronic IL-1 stimulation also mimicked the effect of a DNMT inhibitor on FLS gene expression. Exposure to proinflammatory mediators reversibly alters DNA methylation in FLS by decreasing DNMT expression and function. These data suggest that IL-1 can potentially imprint cells in chronic inflammatory diseases.