Although the biological functions associated with the presence of demethylation-mediated transcriptional activation of the COX-2 gene are not clear, our results showed the DNA hypomethylation of the COX-2 CpG island was involved in the pathogenesis of endometriosis, which also provide an additional evidence that endometriosis ultimately may be an epigenetic disease. Here, PTGS2 is linked to endometriosis.