A 23-year-old female developed progressive cerebellar ataxia as the only presentation of multiple sclerosis for 6 years. Abnormal evoked potentials, an increased oligoclonal band of IgG-K in the cerebrospinal fluid and disseminated white matter lesions in magnetic resonance images (MRI) were compatible with a diagnosis of clinically definite multiple sclerosis. After cyclosporine treatment, the ataxia became stable without further deterioration. A follow-up MRI six months later did not reveal any active lesions. The importance of MRI in the confirmation of the clinical diagnosis and the evaluation of therapeutic effects in patients with atypical multiple sclerosis is emphasized.