Two neuroradiologists reviewed MRI studies of 34 patients with neuro-Behcet's disease (NBD), 22 with multiple sclerosis (MS) and 7 with systemic lupus erythematosus (SLE) with central nervous system involvement, masked to the clinical diagnosis, age and sex of the patients. Of the patients with NBD 12 were in an acute attack; the others had chronic disease. MRI was assessed using a set of criteria, looking at atrophy, the site of discrete parenchymal lesions, regions of predominant involvement and the extent of the lesion(s). The observers also made a guess at the clinical diagnosis. The brain stem and/or basal ganglia were the most predominantly involved sites in all patients with acute NBD; 75% of these lesions were large and confluent, mainly extending from the brain stem to the diencephalon and basal ganglia. However, in chronic cases, the predominant involvement was in the brain stem and/or basal,ganglia in only 36%, and in cerebral hemisphere white matter in another 36%; 27% of these patients showed no parenchymal lesion. Hemisphere white-matter lesions were equally distributed between periventricular and other areas in NBD, while in MS more were periventricular, and in SLE more were nonperiventricular. Brain-stem atrophy was seen in 21% of patients with NBD, with a specificity of 96.5%. In the absence of cortical atrophy, its specificity was 100%. The attempt at making a radiological diagnosis was successful in all cases of acute NBD and 95.5% of patients with MS, but in only 40% of patients with chronic NBD. Most of this latter groups MRI studies were interpreted as MS. An extensive lesion involving the brain stem and basal ganglia seemed to be diagnostic of acute NBD. However, hemisphere white-matter lesions could not be differentiated from those in MS.