There is increasing evidence that there is an antibody mediated, autoimmune component in hereditary and sporadic cerebellar degeneration. The objective of this study is to show clinical improvement or stabilization in ataxia using treatment with intravenous immunoglobulin (IVIG) treatment. This study is a clinical, randomized, placebo-controlled trial of IVIG in adult patients with cerebellar ataxia. Patients will be selected from the motor control clinic based on a diagnosis of sporadic or hereditary cerebellar degeneration. Study drug (placebo versus active) will be given in an intravenous infusion over a 2-day period while the patient is hospitalized. This will be repeated monthly for 2 months. Efficacy measurement will include clinical ataxia rating scales, motor speed testing and clinical gait evaluation. These will be performed at baseline, during each hospitalization, and 1 month following final IVIG infusion. Main outcome measure will be difference from baseline in score on the NINDS ataxia rating scale 1 month following 2 treatments of 2 mg/kg IVIG (total 4 mg/kg).