To identify any relationship between cyclooxygenase-2 expression and the intensity of severe, endometriosisrelated dysmenorrhea. Prospective study. University hospital. Patients with deep endometriosis. During surgery, paired samples of tissue representing deep endometriosis and eutopic endometrium were obtained from 46 patients. Control endometrial tissue samples were obtained from 34 fertile women who underwent laparoscopic tubal ligation or reversal of tubal sterilization. Pain assessment for dysmenorrhea was done with a 10-point linear analogue scale. The percentage of surface immunostained for Cox-2 was determined by an immunohistochemical technique. Relationships between pain score for dysmenorrhea and Cox-2 expression were analyzed. Cox-2 expression was significantly higher in eutopic endometrial stromal cells from patients with deep endometriosis than in stroma from controls during the early, mid, and late secretory phases. In endometriosis patients, Cox-2 expression in eutopic endometrial stromal cells was significantly higher in women with more severe dysmenorrhea (pain score ≥7 vs. < 7) during early and mid secretory phases. Elevated Cox-2 expression in stromal cells in eutopic endometrium from patients with deep endometriosis may play a role in severe, endometriosisrelated dysmenorrhea. To identify any relationship between cyclooxygenase-2 expression and the intensity of severe, endometriosisrelated dysmenorrhea. Prospective study. University hospital. Patients with deep endometriosis. During surgery, paired samples of tissue presenting deep endometriosis and eutopic endometrium were obtained from 46 patients. Control endometrial tissue samples were obtained from 34 fertile women who underwent laparoscopic tubal ligation or reversal of tubal sterilization. Pain assessment for dysmenorrhea was done with a 10-point linear analogue scale. The percentage of surface immunostained for Cox-2 was determined by an immunohistochemical technique. Relationships between pain score for dysmenorrhea and Cox-2 expression were analyzed. Cox-2 expression was significantly higher in eutopic endometrial stromal cells from patients with deep endometriosis than in stroma from controls during the early, mid, and late secretory phases. In endometriosis patients, Cox-2 expression in euto Elevated Cox-2 expression in stromal cells in eutopic endometrium from patients with deep endometriosis may play a role (pain score ≥7 vs. <7) during early and mid secretory phases. in severe, endometriosisrelated dysmenorrhea.