To evaluate the stromal vascularization of different appearances of peritoneal endometriosis, biopsies were taken from peritoneal areas with endometriosis in a series of 135 infertile women and classified as typical (black), red, or white lesions. The number of capillaries per mm2 of stroma, their mean surface area, and the ratio of capillaries/stroma surface area, and the mitotic activity were analyzed in typical, red, and white lesions. Significant differences were found between the different subgroups. The higher vascularization and mitotic activity observed in red lesions suggested the hypothesis that such lesions are very active and probably the first stage of early implantation of endometrial glands and stroma. The poor vascularization and the absence of mitosis observed in white lesions suggested that these lesions are much less active than red lesions and are a quiescent stage of the disease. Our study proves that the "activity" of peritoneal endometriosis is related to the vascularity. This concept must be considered in the further discussion of American Fertility Society Endometriosis Classification. Typical, red and white lesions are three different stages of the peritoneal disease and their relative relation to infertility is also probably different.