Background: Autoantibodies directed against glutamic acid decarboxylase (GAD-Ab) have recently been described in a few patients with progressive cerebellar ataxia, suggesting an autoimmune physiopathologic mechanism. Abstract:Objective: To determine the exact role of GAD-Ab and γ-aminobutyric acid (GABA)-ergic neurotransmission in the pathogenesis of cerebellar ataxia. Design: Case report. Setting: University neurological hospital. Patient: We report the case of a patient with subacute cerebellar ataxia associated with GAD-Ab showing periodic alternating nystagmus (PAN). Intervention: Baclofen, a GABAergic medication, was given to the patient. Main Outcome Measures: Eye movement recording of spontaneous nystagmus and postrotatory vestibular responses. Results: Baclofen was effective in suppressing PAN and improving postrotatory vestibular responses but not for improving cerebellar ataxia. Conclusion: The presence of PAN and the response to baclofen provide a unique opportunity to suggest a direct role of GAD-Ab in cerebellar dysfunction in this patient. Background: Autoantibodies directed against glutamic acid decarboxylase (GAD-Ab) have recently been described in a few patients with progressive cerebellar ataxia, suggesting an autoimmune physiopathologic mechanism. Abstract: Objective: To determine the exact role of GAD-Ab and γ-aminobutyric acid Design: Case report. Setting: University neurological hospital. Patient: We report the case of a patient with subacute cerebellar ataxia associated with GAD-Ab showing periodic alternating nystagmus (PAN). Intervention: Baclofen, a GABAergic medication, was given to the patient. Main Outcome Measures: Eye movement recording of spontaneous nystagmus and postrotatory vestibular responses. Results: Baclofen was effective in suppressing PAN and improving postrotatory vestibular responses but not for improving cerebellar ataxia. Conclusion : The presence of PAN and the response to baclofen provide a unique opportunity to sugg est a direct role of GAD-Ab in cerebellar dysfunction in this patient.