en Original Article Original Article - <strong>Background:</strong> Endometriosis is the presence of endometrial glandular and stromal cells outside of the uterine cavity. Our previous study showed that in vitro culture of human endometrial tissue in a three-dimensional (3D) fibrin matrix could mimic the early stages of endometriosis with invasion gland and stroma formation and sprouting of new vessels.<br> <strong>Objective:</strong> The objective of the present study was to evaluate the expression of aromatase in in vitro cultured endometrial fragments.<br> <strong>Materials and Methods: </strong>Endometrial samples obtained from the fundus of the uterine cavity of ten normal ovulating women on cycle days19-24 that referred to Toronto Center for Advanced Reproductive Technology (T.C.A.R.T) for infertility treatment and cultured in vitro by a three-dimensional fibrin matrix culture system. Cultures were performed in 24-well culture plates.<br> <strong>Results:</strong> After 4 weeks the tissues were fixed and blocked for immunostaining. They were stained by anti-aromatase antibodies. Although a fine anti-glycodelin positive glandular epithelium and anti-cyclooxygenase-2 (COX-2) positive new vessels were detected and reported in our previous study we were unable to detect any aromatase activity in the cultured fragments.<br> <strong>Conclusion:</strong> No aromatase activity was detected in the 3D cultured endometrial tissue. Although it is reported that aromatase is expressed in endometrial disease it is possibly not expressed in this phase in normal endometrium. Endometriosis, In vitro culture, Three-dimensional culture, Aromatase. 105 110 http://ijrm.ir/browse.php?a_code=A-10-549-219&slc_lang=en&sid=1 Jafar Ai jafar_ai@tums.ac.ir Yes Department of Tissue Engineering, Tehran University of Medical Sciences, Tehran, Iran Navid Esfandiari No Division of Reproductive Sciences, Department of Obstetrics and Gynecology, University of Toronto, Toronto, Ontario, Canada Robert Casper No Division of Reproductive Sciences, Department of Obstetrics and Gynecology, University of Toronto, Toronto, Ontario, Canada