Fibromuscular dysplasis of the renal arteries was revealed in 35 patients with renovascular hypertension. Histological examination revealed the following forms of this lesion: fibroplasia of the intima, hyperplasia and fibroplasia of the media, subadventitial fibrosis, as well as their combination. It was found that not only the main branch of the renal artery but also the small intrarenal branches are affected. A decrease of the elastic elements in the middle layer and degenerative changes of various degree in the inner elastic membrane were noted in the wall of the involved artery. This was attended with fibrosis of the intima and disorder of the shape and orientation of the smooth-muscle cells. It is presumed that a local defect in the elastic framework of the artery forms the basis of fibromuscular dysplasia, while the fibrous changes in the vascular wall are evidently secondary and may be associated with the increased synthetic activity of the smooth-muscle cells.