To determine whether treatment with branched-chain amino acids (BCAA) can improve the condition of patients with ataxia, a double-blind crossover study of BCAA therapy was performed in 16 patients with spinocerebellar degeneration (SCD). The patients were treated with BCAA in oral doses of 1.5, 3.0, or 6.0 g or with placebo daily for 4 weeks in each study phase. The order of treatment phases (placebo or BCAA) was assigned randomly. An International Cooperative Ataxia Rating Scale (ICARS) was used to quantify the severity of symptoms of SCD. The mean ICARS score improved significantly with BCAA treatment compared with the mean pretreatment score (p<0.01). In addition, the improvement in the mean global ICARS score was significant in the middle-dose group compared with that in the placebo group (p<0.02). The estimated improvement in kinetic functions compared with pretreatment (p<0.01) was significant after treatment with BCAA, 1.5 and 3.0 g. All of the responders manifested predominantly cerebellar symptoms, especially those with spinocerebellar ataxia type 6 (SCA6). Thus, treatment with BCAA may be effective in patients with the cerebellar form of SCD.