Magnetic resonance (MR) imaging and computed tomography (CT) are useful for the evaluation of central nervous system (CNS) lupus. This report describes the use of cranial MR and CT in 21 patients with systemic lupus erythematosus (SLE) with acute neuropsychiatric symptoms manifested by headache, seizures, focal neurological deficits, psychosis, or organic brain syndrome. Computed tomography was found to be insensitive and detected only diffuse atrophy (two cases), cerebral infarct (one case), and intracerebral haemorrhage (one case) in the 21 patients. Cranial MR images obtained with a General Electric 1.5 tesla Signa unit detected labile and fixed areas of increased proton intensity interpreted as focal oedema (eight cases), infarct (10 cases), haemorrhage (one), atrophy (seven), and acute sinusitis (two). Focal oedema was characterised by labile, high intensity lesions in the gray or white matter of the cerebellum, cerebrum, or brain stem, which completely resolved after aggressive corticosteroid treatment. Most high intensity reversible or fixed lesions evident on MR were not apparent on cranial CT images. In several patients sequential MR images were valuable in monitoring the efforts of treatment. Although histological confirmation of the high intensity brain lesions apparent on MR is desirable, prior necropsy studies suggest that pathological confirmation may be difficult owing to the paucity of recognisable brain lesions in patients with CNS lupus. It is concluded that for the evaluation of acute neuropsychiatric SLE MR is useful and provides more information than cranial CT.