Objective: To evaluate the efficacy of buspirone hydrochloride, a serotonin (5-hydroxytryptamine1A) agonist, in treating patients with cerebellar ataxia. Design: Open-label study in which 20 patients (14 with cerebellar cortical atrophy and six with olivopontocerebellar atrophy) received buspirone hydrochloride, up to 60 mg/d, for 8 weeks. Setting: Research hospital. Main Outcome Measures: Clinical, physiological, and psychological assessment. Results: Nine patients with mild or moderate cerebellar dysfunction who completed the study showed significant improvement in clinical and self-assessment ratings, but not in a motor performance test, posturography (data were incomplete), State-Trait Anxiety Inventory, and Beck Depression Inventory. Seven patients with severe cerebellar dysfunction who completed the study had no improvement on any measure. Conclusions: Buspirone may be effective in treating mild to moderate cerebellar ataxia. A double-blind study of the efficacy of buspirone in cerebellar ataxia is warranted.