We report the case of an intrapericardial lesion with the pathological features of a multilocular cyst, found on the adventitia of an aortic aneurysm in a 62-yr-old man. Microscopically, it consisted of numerous cavities lined by flattened or cuboidal cells with the immunocytochemical and electron microscopical features of mesothelium, sometimes forming a hyperplastic pattern. Numerous nerve bundles and collections of inflammatory cells were present in the stroma. These features were consistent with a reactive lesion.