Many of the proinflammatory effects of interleukin (IL)-1β and tumor necrosis factor-α (TNF-α) are mediated through cyclooxygenase-2 (COX-2)-dependent prostaglandin E2 (PGE2). The purpose of this study was to examine the expression of IL-1β and TNF-α in COX-2–deficient osteoblasts during inflammation. Primary osteoblasts prepared from wild-type (WT) and COX-2 knockout (K/O) mice were exposed to lipopolysaccharide (LPS) and endodontic obturation materials. An enzyme-linked immunosorbent assay was used to measure cytokine levels. LPS treatment led to a significant upregulation in IL-1β and TNF-α levels in both WT and K/O cells. TNF-α upregulation in response to LPS was much more pronounced in K/O cells compared with WT cells (p < 0.05). All materials tested except for gutta percha caused an increase in IL-1β expression. In conclusion, there appears to be a positive feedback regulation between TNF-α and COX-2–dependent PGE2 during LPS-induced inflammatory reactions.