Aquaporin-1 (AQP1) is involved in the angiogenesis and structural modifications of microvessels and possibly also in the pathogenesis of idiopathic menhorrhagia, where a reduced AQP1 expression is seen in the endometrium. Mifepristone treatment induces reduced menstrual bleeding and amenorrhea and also has a direct effect on endometrial arterioles. Administered with gestagen-only contraceptive methods, antiprogestins improve the bleeding pattern. The objective of this study was to evaluate the AQP1 expression in endometrial blood vessels during normal cycle and after mifepristone treatment. Localization and expression of AQP1 was determined using immunohistochemistry and reverse transcriptase chain reaction (RT-PCR) in 43 biopsies from human endometrium taken during a normal cycle and after mifepristone treatment. AQP1 expression in human endometrial vessels is not cycle dependent and is stronger in capillaries and arteries than in veins. After mifepristone treatment the staining intensity was increased, but not the number of stained vessels. The presence of AQP1 was also confirmed using RT-PCR. The changes in AQP1 expression could contribute to the reduced bleeding seen following mifepristone treatment and could be an effect of either antagonizing progesterone or cortisol.