The purpose of this study is to evaluate the level and pattern of cyclooxygenase-2 (Cox-2) expression in endometriosis and to investigate the correlation between the expression of Cox-2 and several clinicosurgical parameters. Twenty-six patients with endometriosis and 21 patients with other benign gynecological conditions were enrolled. The eutopic endometrium was sampled by pipelle, and fragments of ovarian endometrioma and non-endometriotic ovarian cysts were sampled during surgery. Total RNA isolation and semiquantitative reverse transcriptase polymerase chain reaction was performed. The expression of Cox-2 mRNA (mean +/- SEM) in eutopic endometrium and ovarian endometriotic tissue significantly increased in patients with endometriosis compared with the controls. The expression of Cox-2 increased significantly in the proliferative phase in eutopic endometrium and in the secretory phase in ovarian endometriotic tissue of patients with endometriosis compared with the controls. Cox-2 mRNA expression in the endometrium and ovarian lesions significantly correlated with serum CA-125 and the diameter of the endometrioma. Cox-2 expression increased in the eutopic endometrium and ovarian endometriotic tissue of the patients with endometriosis. These findings indicate that Cox-2 may be involved in the pathogenesis and progression of endometriosis. Copyright (c) 2009 S. Karger AG, Basel.