Magnetic resonance imaging (MRI) was performed in 25 patients with Behçet's disease, of whom 15 had clinical involvement of the central nervous system (CNS). Brain MRI was abnormal in 11/15 with and in 1/10 without CNS involvement. The most common sites of MR lesions were the cerebral white matter and brainstem. Lesions were also seen in the basal ganglia, cerebellum, optic nerves and spinal cord. The cerebral white matter lesions were always asymptomatic and usually small, whereas the brainstem lesions were often symptomatic and sometimes extensive. Follow-up MRI in 2 patients presenting with brainstem syndromes and treated with immunosuppression showed resolution of a large lesion in 1 patient in whom there was clinical remission, and the development of marked brainstem atrophy in the other who became severely disabled. The pattern of MRI abnormalities may assist the diagnosis and suggests a role for MRI in monitoring the effect of treatment in patients with neurological Behçet's disease.