A 63-year-old man complaining of prolonged imbalance underwent 11C-CB184 PET to assess microglial activation 3 years after being diagnosed with cerebellar ataxia associated with HIV infection. 11C-CB184 images revealed significant cerebellar uptake where MRI signal abnormalities were observed at disease onset, although these abnormalities had mostly disappeared at the time of 11C-CB184 PET. Microglia are believed to be a long-term reservoir for HIV infection, causing persistent immune activation (ie, chronic inflammation). Hence, in this case, increased 11C-CB184 binding may reflect persistent microglial activation along with HIV persistence in the cerebellum. However, further pathological investigations are desired to validate 11C-CB184 PET.