Arterial fibrodysplasia affected 196 patients (172 females, 24 males) harboring a total of 316 diseased vessels. Renal artery lesions were documented in 152 adult and 25 pediatric patients. Superior mesenteric, celiac, common hepatic, and external iliac arteries were occasionally involved. Intimal fibroplasia and medial hyperplasia are uncommon types of arterial dysplasia. Medical fibroplasia represents a continum of disease, including pathologic processes heretofore categorized as subadventital or perimedial fibroplasia. A previously unrecognized form of perimedial dysplasia, characterized by dense accumulations of elastic tissue about the periphery of the media, is reported as a distinct pathologic entity. Hormonal influences, traction-stretch stresses, and peculiarities in distribution of vasa vasorum in vessels affected by dysplastic processes may be prerequisite to the evolution of arterial fibrodysplasia.