Adult periodontitis is a chronic destructive disease characterized by an interaction between gram-negative bacteria and the host inflammatory response. Microbial substances such as lipopolysaccharide can activate host cells, e.g., macrophages, fibroblasts and keratinocytes, to secrete proinflammatory cytokines including tumor necrosis factor α and interleukin 1β (IL-1β). This study examined the hypothesis that periodontitis tissue contains increased levels of cytokines that promote osseous and connective tissue destruction. To test this hypothesis, diseased and healthy gingival biopsies were examined for differences in the expression of cytokine mRNA for the pro-inflammatory cytokines tumor necrosis factor α and IL-1β and the anti-inflammatory cytokine IL-1ra using quantitative reverse transcriptase polymerase chain reaction and in situ hybridization methods. The levels of tumor necrosis factor α and IL-1ra mRNA were shown to be significantly higher in diseased than healthy tissues. Additionally, a significantly correlated expression of IL-lβ and IL-1ra mRNA was seen in all tissue examined. Analysis of tissue sections by immunohistochemical and in situ hybridization techniques revealed a mononuclear cell infiltrate that consisted of a higher average number of cells staining positive for tumor necrosis factor α mRNA, CD14, and CD3 in the diseased than healthy tissues. Although both diseased and healthy tissues expressed IL-1β and IL-1ra mRNA in the epithelium, the diseased tissue biopsies expressed more IL-1β and IL-1ra mRNA in the connective tissue. These results implicate the potential involvement of both the pro- and anti-inflammatory cytokines in the regulation of the chronic inflammatory disease adult periodontitis.