The occurrence of vascular involvement in adenomyosis has been noted earlier, but there has been little detailed study of this phenomenon. In this study, we examined a large series of uteri with adenomyosis (n=434), specifically looking for the presence of adenomyotic foci within vascular channels. Vascular involvement was identified in 54 cases (12.4%). In 19 of 54 cases (35%) a single vessel was involved, in 16 cases (30%) 2 to 3 vessels, and in 19 cases (35%) multiple vessels. In 34 cases (63%), the intravascular component comprised endometrial stroma only and in 20 cases (37%) there was a mixture of glands and stroma, although even in these cases some of the individual foci exhibiting vascular involvement consisted of stroma only. In most cases, the intravascular component protruded into the vessel lumen beneath an intact endothelial lining. Immunohistochemistry performed in a small number of cases confirmed the intravascular location and showed the intravascular tissue to be lined on the luminal surface with endothelial cells. Pathologists should be aware of the phenomenon of vascular involvement in adenomyosis, which is relatively common and, when widespread, may result in the consideration of a neoplastic process. The pattern of vascular involvement raises the possibility that adenomyosis develops from cells intimately associated with myometrial blood vessels, perhaps multipotential perivascular cells.