Among patients who underwent cerebrospinal fluid (CSF) and magnetic resonance imaging (MRI) examination during a 5-year period, we found 18 patients at the multiple sclerosis center of the University of Rome and clinically definite multiple sclerosis, MRI white matter abnormalities, normal CSF examination, a disease duration of at least 1 year and an upper age of < 45 years at onset These patients were examined again with a variety of tests screening for different diseases mimicking multiple sclerosis. Alternative diagnoses reached after laboratory tests were: Lyme disease, two cases of vasculitis, mitochondrial encephalomyopathy, multiple ischemic lesions caused by atrial septum aneurysm and olivopontocerebellar atrophy. Hence, six of these 18 patients had a final diagnosis other than multiple sclerosis, while 12 remained with a final diagnosis of "MS with normal CSF". Our study suggests that in patients with a clinical picture of multiple sclerosis and disseminated white matter MRI lesions but no CSF abnormalities, the classical clinical criteria may not be sufficiently specific, unless confirmed by a very prolonged clinical history with repeated MRI.