We report two cases of the moyamoya syndrome which became clinically apparent after irradiation of an optic glioma during childhood. A summary of 14 cases of this syndrome following irradiation of intracranial tumors is also presented. Nine of these cases were optic gliomas; five were found in children with neurofibromatosis, another disorder that has a strong association with the moyamoya syndrome. The effectiveness of irradiation of optic gliomas in childhood is not definitely established. The possibility of inducing serious vascular disease is a further reason for caution when considering irradiating these tumors.