Two observers, blinded to the patient's neurological status, reviewed 134 MRI studies of 98 consecutive patients with Behcet's disease (BD), to define imaging patterns and to look for any relationship between the MRI findings and the timing of the examination in patients with differing courses of disease. There were 43 patients with overt parenchymal central nervous system (CNS) involvement, 22 with attacks and remissions, 15 with secondary progressive and six with primary progressive disease; 14 had raised intracranial pressure (RICP). Of the remaining 41 patients without specific neurological complaints, 16 had abnormalities on examination (silent CNS involvement) and 25 did not. During an acute CNS attack, the most common finding was a large lesion in the brain-stem or basal ganglia, extending to the diencephalon. On MRI performed after remission of an acute attack or during secondary progression, the same sites were affected, but the lesions were smaller or scattered, with less clearly defined margins. In primary progressive disease or silent CNS involvement, the cerebral white matter was most commonly involved, but almost half the MRI studies were normal. The brain parenchyma was abnormal in only one of the patients with RICP. MRI was normal in all but three patients without clinical CNS involvement, in whom it showed a few millimetric white-matter lesions. Brain-stem atrophy was seen in 15 patients examined >1 year after an initial parenchymal CNS episode, with secondary progressive cases predominating.