Ten patients with clinically isolated idiopathic retinal vasculitis who had a positive family history for multiple sclerosis (MS) or positive typing for HLA B7 underwent magnetic resonance imaging (MRI) of brain and optic nerves in order to establish the frequency of clinically silent lesions. Brain MRI was normal in seven and abnormal in three: one had a single small white matter lesion, two had extensive white matter abnormalities resembling those seen in MS. In two patients a lesion was shown in the optic nerve. These findings suggest that a minority of patients with idiopathic retinal vasculitis have disseminated central nervous system lesions characteristic of MS, the frequency of such changes being less than in patients with isolated optic neuritis.