We noticed a blue-line in the endometrial cavity between the tubal ostiae after injection of methylene blue (to determine tubal patency). We have seen this “blue-line” even in cases with normal or unicornuate uterus and/or in cases with patent or occluded fallopian tubes(Picture 1). So the be...st explanation of this finding may be the high speed jet or turbulence of dye in the top or the deepest part of endometrial cavity. We simply postulated that the zone which holds the methylene blue is the zone where the flashing dye strikes vertically over there and the dye penatrates into the endometrial epithelium and glands. We used this line as a guide that shows midline during operative hysteroscopy ( especially in cases with septate uterus) and we don’t ecxatly know reason why it occurs. It is necessary to perform histologic, molecular or clinical studies on this subject. It may have a multifactorial aetiology. We performed a prospective case control study and will publish it soon after when we get the results.