Objective To study the role of aromatase cytochrom P450 in the pathogenesis of endometriosis and the practicability of it serving as a diagnostic marker for endometriosis in infertility. Methods The expression of aromatase cytochrom P450 in eutopic endometrium of endometriosis and non-endometriosis of infertility was determined by immunohistochemical assay.Results There was a significant difference in the expression of aromatase cytochrom P450 between eutopic endometrium of endometriosis and non-endometriosis.There was no difference between proliferative and secretory endometrium of endometriosis.No correlation was observed between the expression of aromatase cytochrome P450 and AFS stage of endometriosis.As a diagnostic test for endometriosis by measuring aromatase cytochrome P450 in eutopic endometrium of endometriosis in infertility,the sensitivity,specificity,positive predictive value and negative predictive value were 90%,86%,86.5% and 89.6%.Conclusion The expression of aromatase cytochrom P450 in endometrium may play an important role in pathogenesis of endometriosis and can be used as a diagnostic test for endometriosis in infertility.