Central Nervous System involvement in Monoclonal gammopathies of undetermined significance has seldomly been reported and in all the cases a demyelinating disease was found. We report the case of a young man who had been suffering for five years of progressive cerebellar syndrome. MRI showed marked cerebellar vermis atrophy. An IgG lambda monoclonal gammopathy was revealed in the serum. Cerebrospinal fluid examination showed oligoclonal bands and elevated Link-Index. Serologic research for HBV, HCV, HIV, Lues, Rubella, Measles was negative, as also genetic analysis for SCA1, SCA2, SCA3, SCA7 and Friederich's ataxia. Nerve conduction studies were normal. Plasmatic vit.E was low, but treatment with high doses of tocopherol was ineffective. i.v. immunoglobulins and steroids obtained only transient clinical benefits. In conclusion, we hypothesize a pathogenetic role of the IgG in this cerebellar atrophy.