Uterine hemorrhages are dependent on the vascular architecture of the endometrium: the endometrial spiral arteries supply sectors strictly separated from each other at intervals of about 1.4 mm. Their function is so closely coupled with the endocrine regulation of the cycle that the presence of hormonal receptors in the vessel wall must be assumed. Menstrual bleeding is an instructive example of morphologically detectable endocrine controlled processes. Endometrial hemorrhage in adenocarcinoma appears early, but it is not an early symptom because the surface epithelium remains intact at first. The diagnosis of precancerous changes in the endometrium (adenomatous hyperplasia--carcinoma in situ) is based on the condition of the surface epithelial cells. Extrauterine pregnancies, miscarriages and trophoblastic diseases are usually diagnosed from the uterine hemorrhage. The quality of the hemorrhage in early pregnancy has less diagnostic importance than, for example, in dyshormonal hemorrhagic disorders.