It is widely accepted that magnetic resonance imaging (MRI) findings are not totally specific for the diagnosis of multiple sclerosis. White matter lesions that mimic those of multiple sclerosis may be detected in both normal volunteers and patients harbouring different diseases. Virtually all the characteristic features of multiple sclerosis are sometimes encountered in other conditions affecting predominantly the white matter. Different conditions such as vasculitis, subcortical atherosclerotic leukoencephalopathy, Lyme disease, or acute disseminated encephalomyelitis can be virtually indistinguishable from multiple sclerosis on conventional MR images. Also the FLAIR technique adds little to the differential diagnosis. The calculation of magnetisation transfer ratio (MT ratio) may be useful to better characterise some entities, such as vasculitis, from multiple sclerosis.