Objective: To investigate the expression of aquaporin (AQP) in eutopic and ectopic endometrial tissues from women with endometriomas. Design: Controlled laboratory research. Setting: Hospital-based unit for gynecology and obstetrics and research laboratories. Patient(s): Premenopausal women undergoing laparoscopy for endometriomas. Intervention(s): Endometrial biopsy samples obtained from 70 women with endometriomas. Main Outcome Measure(s): Semiquantitative analysis by immunohistochemistry. Result(s): Aquaporins 2, 5, and 8 were mainly located in luminal and glandular epithelia. The frequency of positive immunostaining for aquaporins 2, 5, and 8 decreased in ectopic compared with eutopic endometria. Aquaporins 2, 5, and 8 were found at a low frequency in the endometria in early proliferative phases but at a higher frequency in late proliferative and secretory phases. There were no significant differences in the menstrual cycle of the proliferative phase and secretory phase in the two groups. Conclusion(s): Aquaporins 2, 5, and 8 were expressed with greater frequency in eutopic endometrial cells than inectopic endometrial cells, suggesting that eutopic endometrial cells have stronger migration activity than ectopic endometrial cells in women with endometriosis. Copyright © 2010 American Society for Reproductive Medicine, Published by Elsevier Inc.