Imaging of type 1 metabotropic glutamate receptor (mGluR1), which is predominantly expressed in Purkinje cells, is currently possible using positron emission tomography (PET). We have previously shown that cerebellar mGluR1 availability decreases in patients with cerebellar ataxia and that the degree of ataxia is related to the magnitude of cerebellar mGluR1 availability. This study presents the longitudinal changes in cerebellar mGluR1 availability after an interval of years in patients with spinocerebellar ataxia type 6 (SCA6).Three patients with SCA6 underwent baseline and follow-up mGluR1 PET and magnetic resonance imaging (MRI) at intervals of 2-6 years. The volume-of-interest was placed on the cerebellum. The binding potential (BPND) was calculated to estimate the mGluR1 availability using a simplified reference tissue model. Cerebellar volume was measured using MRI. The degree of ataxia was scored with the Scale for the Assessment and Rating of Ataxia (SARA).The cerebellar BPND values, cerebellar volume, and SARA scores changed by 2.73-7.49 %, 0.60-1.75 %, and 0.5-3, respectively, from baseline to follow-up assessments. The rates of reduction in cerebellar mGluR1 availability were higher than those in cerebellar volume.This study shows that cerebellar mGluR1 availability decreases with disease progression in SCA6, and that mGluR1 imaging is likely superior to MRI in identifying small changes in the cerebellum. Therefore, mGluR1 imaging can be used as a specific biomarker for monitoring cerebellar function.