We assessed the clinical outcome after coenzyme Q(10) (CoQ(10)) therapy in 14 patients presenting ataxia classified into two groups according to CoQ(10) values in muscle (deficient or not). We performed an open-label prospective study: patients were evaluated clinically (international cooperative ataxia rating scale [ICARS] scale, MRI, and videotape registration) at baseline and every 6 months during a period of 2 years after CoQ(10) treatment (30 mg/kg/day). Patients with CoQ(10) deficiency showed a statistically significant reduction of ICARS scores (Wilcoxon test: P = 0.018) after 2 years of CoQ(10) treatment when compared with baseline conditions. In patients without CoQ(10) deficiency, no statistically significant differences were observed in total ICARS scores after therapy, although 1 patient from this group showed a remarkable clinical amelioration. Biochemical diagnosis of CoQ(10) deficiency was a useful tool for the selection of patients who are good candidates for treatment as all of them responded to therapy. However, the remarkable clinical response in 1 case without CoQ(10) deficiency highlights the importance of treatment trials for identification of patients with CoQ(10)-responsive ataxia.