JC virus (JCV) is a human polyomavirus that infects the central nervous system (CNS) of immunocompromised patients. JCV granule cell neuronopathy (JCV-GCN) is caused by infection of cerebellar granule cells, causing ataxia. A 77-year-old man with iatrogenic lymphopenia presented with severe ataxia and was diagnosed with JCV-GCN. His ataxia and cerebrospinal fluid (CSF) improved with intravenous immunoglobulin, high-dose intravenous methylprednisolone, mirtazapine, and mefloquine. Interleukin-7 (IL-7) therapy reconstituted his lymphocytes and reduced his CSF JCV load. One month after IL-7 therapy, he developed worsening ataxia and CSF inflammation, which raised suspicion for immune reconstitution inflammatory syndrome. Steroids were restarted and his ataxia stabilized.