['To investigate whether WT1 protein expression is altered in eutopic endometrium of endometriosis patients.', 'Prospective study.', 'University hospital.', 'Patients with endometriosis and fertile women with macroscopically normal pelvic cavities.', 'During surgery, endometrial tissues were obtained from 59 patients with deep infiltrating endometriosis, ovarian endometriosis, or only superficial peritoneal endometriosis. Control endometrial tissue samples were obtained from 40 fertile women who underwent laparoscopic tubal ligation or reversal of tubal sterilization.', 'The percentage of nuclear surface positively immunostained for WT1 (PI) and WT1-positive microvessel density.', 'The PI was significantly lower in endometrial stromal cells from patients with deep infiltrating endometriosis than in stroma from control subjects during the midsecretory phase, whereas there was no significant difference between the two populations during the proliferative, early secretory, and late secretory phases. The PI was also significantly lower during the midsecretory phase in endometrial stromal cells from patients with ovarian endometriosis and superficial peritoneal endometriosis compared with control subjects.'] Is expression of WT1 down-regulated in eutopic endometrium obtained during the midsecretory phase from patients with endometriosis?