Abstract The natural course of fibromuscular dysplasia (FMD) of the internal carotid artery (ICA). a stenosing angiopathy associated with cerebrovascular insufficiency, has not been described. A search of medical records located 16 female patients with angiographically demonstrated FMD of the ICA. The identical twin of 1 patient was included in the registry on the basis of noninvasive studies consistent with FMD of the ICA. The mean age at diagnosis was 58 years. Follow-up examinations were performed an average of 3.8 years after diagnosis (range, 1 to 9 years); the evaluation included clinical, angiographic, and Doppler studies. Fifteen patients showed no evidence of progression of FMD, whereas 2 patients with coincident atherosclerotic disease had suffered strokes. One patient had undergone surgical dilatation of the ICA. 3 had received oral anticoagulants, and 13 had received either aspirin or no specific therapy. In light of the apparently benign clinical course of uncomplicated FMD of the ICA, it is concluded that dilatation is rarely warranted.