Recently, an inducible microsomal human prostaglandin E synthase (mPGES) was identified. This enzyme converts the cyclooxygenase (COX) product, prostaglandin (PG) H2, to PGE2, an eicosanoid linked to carcinogenesis. Although elevated levels of PGE2 have been observed in many tumor types including colorectal adenomas and cancers, its role in the pathophysiology of endometriosis is unknown. We previously reported increased expression of COX-2 messenger RNA (mRNA) in local lesions of endometriosis. To further elucidate the mechanism responsible for the elevated levels of PGE2 in endometriosis, we examined the expression levels of mPGES.