Purpose: Diabetes mellitus is a clinically and genetically heterogeneous group of metabolic disorders manifested by abnormally high levels of glucose in the blood. Mounting evidence demonstrates that diabetes is a risk factor for gingivitis and periodontitis. The circulating mononuclear phagocytes in diabetic patients with hyperglycemia are chronically exposed to high level of serum glucose. Thus, this study attempted to determine the effect of pre-exposure of monocytes and macrophages to high concentration of glucose on lipopolysaccharide (LPS)-induced production of pro-inflammatory mediators. Material and Methods: For this purpose, cells were cultured in medium containing normal (5 mM) or high glucose (25 mM) for 4-5 weeks before treatment for 24 h with LPS. LPS was highly purified from Porphyromonas gingivalis or Prevotella intermedia by phenol extraction. Result: Results showed that prolonged pre-exposure of cells to high glucose markedly increased LPS-stimulated NO secretion when compared to normal glucose. In addition to NO, high glucose also augmented LPS-stimulated IL-6, IL-8, and TNF-$alpha$ secretion after cells were exposed to high glucose for 4 weeks. Conclusion: The present study demonstrates that pre-exposure of mononuclear phagocytes with high glucose augments LPS-stimulated production of pro-inflammatory mediators. These findings may explain why periodontal tissue destruction in diabetic patients is more severe than that in non-diabetic individuals.