Aberrant production of cyclooxygenase-2 (COX-2) plays pivotal roles in many pathological processes including tumorigenesis and endometriosis, although the underlying mechanism remains obscure. Herein we report evidence to demonstrate that COX-2 is distinctly regulated by IL-1β in normal and endometriotic stroma. Ectopic endometriotic stromal cell is at least 100 times more sensitive to IL-1β treatment, compared with its eutopic counterpart. Induction of COX-2 expression in normal endometrial stroma by IL-1β is primary due to enhancement of COX-2 mRNA stability. In contrast, IL-1β not only increases COX-2 mRNA stability but also up-regulates COX-2 promoter activity in ectopic endometriotic stroma. Induction of COX-2 promoter activity by IL-1β is mediated via MAPK-dependent phosphorylation of cAMP-responding element binding protein. Promoter activity and EMSAs demonstrate that a cAMP response element site located at −571/−564 of COX-2 promoter is critical for IL-1β-induced COX-2 gene expression. Our results indicate that elevation of COX-2 expression in endometriotic tissues may result from increased sensitivity to proinflammatory cytokines such as IL-1β, which is consistently present in the peritoneal fluid of endometriosis patients. Distinct regulation of COX-2 gene by IL-1β may play a critical role in pathophysiological processes such as cancer formation and endometriosis.