Machado-Joseph Disease (MJD/SCA3) is an autosomal dominant spinocerebellar degeneration that evolves to disability and death. Experimental data have shown that serotonin is an important cerebellar neurotransmitter and that impairment of the serotoninergic cerebellar system can induce cerebellar ataxia. To evaluate the efficacy of fluoxetine, a serotonin reuptake inhibitor, in treating neurologic dysfunction in patients with MJD. Thirteen MJD patients were treated with fluoxetine (20 mg/day) and were followed-up for 6 weeks. Outcome measures included functional capacity, standardized neurologic and cognitive ratings. The Montgomery-Asberg depression rating scale was used to control depressive symptoms. There was no significant improvement in motor abilities after 6 weeks of treatment. These results suggest that fluoxetine has no benefit in motor function of patients with MJD/SCA3.