Aneurysms of both common carotid arteries were seen in a patient with neurofibromatosis. Histologic examination of the involved arteries demonstrated a dense proliferation of spindle cells within the arterial walls accompanied by medial cystic degeneration and disruption of the tunica elastica. These changes were also observed in adjacent, grossly uninvolved segments of the blood vessels. Immunohistochemical analysis of the proliferating cells was positive with antibodies against desmin and muscle-specific actin. The findings demonstrate and confirm the smooth muscle origin of the aberrant proliferated cell population in arteries in a patient with von Recklinghausen's neurofibromatosis.