Objective - To investigate the possible abnormal regional brain metabolism during ataxic gait in olivopontocerebellar atrophy (OPCA), and to evaluate the response of the cerebellar subregions to instability during bipedal gait. Material and methods - On 9 patients with OPCA in early phase and on 10 age-matched normal subjects, we performed positron emission tomography (PET) with 2-[18 F]fluoro-2-deoxy-D-glucose (FDG) under two different conditions: supine resting and 30 min treadmill walking. Results - Both in normals and in patients with OPCA, the FDG uptake in the walking state (Uwalk) was significantly greater than that in the resting state (Urest) in the pyramis, declive-folium-tuber and culmen of the cerebellar vermis, and in the thalamus. In the patients, the Uwalk was also significantly greater than the Urest in the posterior lobe of cerebellar hemisphere and in the pons and midbrain. In the pyramis, the activation ratio (=Uwalk/Urest) of the patients was significantly lower than that of the normals. Conclusions - We considered that these findings reflect the pathophysiology of ataxic gait in OPCA patients and the compensatory mechanism for the instability during ataxic gait.