Purpose: In addition to regulating body weight, leptin is also recognized for its role in the regulation of immune function and inflammation. The purpose of this study was to investigate the effect of leptin on Prevotella (P.) intermedia lipopolysaccharide (LPS)-induced tumor necrosis factor (TNF)-$alpha$ production in differentiated THP-1 cells, a human monocytic cell line. Methods: LPS from P. intermedia ATCC 25611 was prepared by the standard hot phenol-water method. THP-1 cells were incubated in the medium supplemented with phorbol myristate acetate to induce differentiation into macrophage-like cells. The amount of TNF-$alpha$ and interleukin-8 secreted into the culture medium was determined by enzyme-linked immunosorbent assay (ELISA). TNF-$alpha$ and Ob-R mRNA expression levels were determined by semi-quantitative reverse transcription-polymerase chain reaction analysis. Results: Leptin enhanced P. intermedia LPS-induced TNF-$alpha$ production in a dose-dependent manner. Leptin modulated P. intermedia LPS-induced TNF-$alpha$ expression predominantly at the transcriptional level. Effect of leptin on P. intermedia LPS-induced TNF-$alpha$ production was not mediated by the leptin receptor. Conclusions: The ability of leptin to enhance P. intermedia LPS-induced TNF-$alpha$ production may be important in the establishment of chronic lesion accompanied by osseous tissue destruction observed in inflammatory periodontal disease.