Objectives. To study the changes in aromatase, Ki-67 and cyclooxygenase-2 (COX-2) expression during the menstrual cycle in both endometrial polyps and normal endometria. Patients and methods. Paraffin-embedded tissue samples from 118 premenopausal patients were submitted to immunohistochemistry for measurement of aromatase, COX-2 and Ki-67 expression. Fifty cases of endometrial polyps and 68 cases of disease-free endometrium were included. Results. The presence of aromatase expression was significantly higher in endometrial polyps than in disease-free endometria. On the other hand, changes in COX-2 and Ki-67 expression followed a similar pattern during the menstrual cycle in both groups, expression peaking during the proliferative phase and falling during the late luteal phase. Conclusion. A significantly higher proportion of endometrial polyps express aromatase compared with disease-free endometrium; however, no correlation was found between aromatase expression and changes in either Ki-67 or COX-2 expression during the menstrual cycle.