Fifty patients with clinical diagnoses of disease within the posterior fossa were examined with a nuclear magnetic resonance (NMR) scanner and the results were compared with X-ray computed tomography (CT). A variety of NMR pulse sequences reflecting proton density, T1, T2 and blood flow were used and imaging was performed in transverse coronal and sagittal planes. In many cases NMR provided diagnostic information unavailable from CT scans. This included recognition of lesions not seen with CT in infarction and multiple sclerosis, as well as more precise definition of mass effects, oedema and anatomical relationships in other diseases. Extrinsic and intrinsic tumours were readily distinguished, as were brain-stem and cerebellar tumours. Arnold-Chiari malformations were demonstrated with sagittal scans and cerebellar atrophy was identified. A patient treated with radiotherapy displayed more extensive changes in the surrounding brain with NMR than with CT. Evidence of occlusion or reduced flow was found in two cases with vertebro-basilar disease. Occlusion of the internal carotid artery was also demonstrated. Computed tomography was superior to NMR in demonstrating calcification and bone erosion. In four of 14 tumours studied, contrast-enhanced CT demonstrated the margin between tumour and surrounding oedema or brain better than NMR. Nuclear magnetic resonance imaging is a versatile, non-invasive technique capable of demonstrating a wide spectrum of disease within the posterior fossa.