To analyze the expression of vascular endothelial growth factor A (VEGF- A) and receptors (VEGFR- 1 and VEGFR- 2) in endometrial blood vessels, as well as microvascular density (MVD), in endometrial biopsy samples from idiopathic menorrhagia patients. Design: Prospective clinical study. Setting: University hospital, unit of gynecology. Patient(s): Twenty-four patients with idiopathic menorrhagia and 18 healthy fertile women. Intervention(s): Blood sampling for hormone measurement, hysteroscopy, and endometrial biopsy sampling. Endometrial biopsy samples were used for immunohistochemistry assessments and image analysis of stained endothelial structures for VEGF- A, VEGFR- 1, VEGFR- 2, and CD34. Main Outcome Measure(s): Appearance of the endometrial vascular immunoreactivity for VEGF- A, VEGFR- 1, and VEGFR- 2, MVD and computer-assisted stereological analysis of immunoassayed blood vessels. Result(s): Although the MVD did not differ between patients and controls, we observed that vascular expression of VEGF- A, VEGFR- 1, and VEGFR- 2 in capillaries was 1.8- fold, 1.8- fold, and 2.0- fold higher, respectively, in the menorrhagia group when assessed as the number of stained capillaries per unit area. There were also a twofold higher number of arterioles, which were VEGFR- 2 positive in the menorrhagia group. Conclusion(s): Up-regulation of VEGF- A and receptors VEGFR- 1 and VEGFR- 2 in capillaries in menorrhagia could be involved in abnormal endometrial vascular structure and permeability. To analyze the expression of vascular endothelial growth factor A (VEGF- A) and receptors (VEGFR- 1 and VEGFR- 2) in endometrial blood vessels, as well as microvascular density (MVD), in endometrial biopsy samples from idiopathic menorrhagia patients. Design Patient (s): Twenty-four patients with idiopathic menorrhagia and 18 healthy fertile women. Intervention (s): Blood sampling for hormone measurement, hysteroscopy, and endometrial biopsy sampling. Prospective clinical study. Setting: University hospital, unit of gynecology. Endometrial biopsy samples were used for immunohistochemistry assessments and image analysis of stained endothelial structures for VEGF-A, VEGFR-1, VEGFR-2, and CD34. Main Outcome Measure (s): Appearance of the endometrial vascular immunoreactivity for VEGF- A, VEGFR - 1, and VEGFR-2, MVD and computer-assisted stereological analysis of immunoassayed blood vessels. Result (s): Although the MVD did not differ between patients and controls, we observed that vascula r expression of VEGF-A, VEGFR-1, and VEGFR-2 in capillaries was 1.8-fold, 1.8-fold, and 2.0-fold higher, respectively, in the menorrhagia group when assessed as the number of stained capillaries per unit area. Conclusion (s): Up-regulation of VEGF-A and receptors VEGFR-1 and VEGFR-2 in capillaries in menorrhagia could be involved in abnormal endometrial vascular structure and permeability.