A prospective clinical study was carried out to investigate whether endometrial microvessels in patients with idiopathic heavy menstrual bleeding (HMB) of endometrial origin (HMB-E) are fragile due to low pericyte coverage. Idiopathic HMB-E is characterized by large endothelial cell gaps related to the microvascular overexpression of vascular endothelial growth factor (VEGF)-A and VEGF receptors 1-3. A total of 10 women with a normal menstrual cycle and a history of HMB of <5 years, and 17 healthy women with a normal menstrual cycle were recruited from the Karolinska University Hospital. Blood samples were obtained for hormone analysis and coagulation tests. Endometrial biopsies were collected in the proliferative or in the secretory phase. Pericyte coverage was assessed using immunohistochemical staining for smooth muscle actin-α (SMAα) and by image analysis (microvascular density) of endometrial biopsies from 10 patients with HMB-E and 17 healthy ovulating women (control subjects). Previously published data on endothelial cell gap size and the expression of VEGF receptors were used. Although microvascular density did not differ between the patients with HMB-E and the control subjects, the number of SMAα-positive microvessels in the proliferative phase was significantly (P=0.005) lower in the patients with HMB-E than in the control subjects. Moreover, the number of SMAα-positive microvessels in the control subjects was significantly fewer in the secretory (P=0.04) than in the proliferative phase, whereas this number did not differ among the patients with HMB-E regardless of phase. A significant negative correlation was observed between the number of VEGF-A-positive microvessels and microvessels with pericyte coverage (r=0.8; P=0.04). Finally, the endothelial cell layer was significantly thicker in the patients with HMB-E than in the control subjects. Thus, the upregulation of VEGF-A in idiopathic HMB-E is associated with a low pericyte coverage during the proliferative phase of intense angiogenesis, which may confer vessel fragility, possibly leading to excessive blood loss.