We evaluated angiogenesis in eutopic endometrium and ectopic endometrium from patients with endometriosis. Microvessels were identified by immunohistochemistry using anti-von-Willebrand factor antibody and vascular parameters were measured using a computerized image analysis system. No relationship was observed between vascular density and type of endometriotic lesion. Heterogeneity in vascular density was observed in different lesions within the same patient. Between red and black peritoneal lesions a significant difference was observed in the frequency distribution of luminal diameter. In black lesions the variability of luminal diameter was decreased and most microvessels were less than 20 mm in diameter. We suggest that the frequency distribution of microvessel luminal diameter of red and black peritoneal lesions may be different and should be important when studying the regulation of angiogenesis in endometriotic implants.