Granule cell neuronopathy (GCN) is a rare presentation of JC virus (JCV) infection targeting the cerebellum.1 To date, 2 cases have been described in patients prescribed natalizumab for MS.2,3 Recent case reports have acknowledged the importance of controlled reconstitution of host immunity in treating JCV-related neurologic disorders.4 In particular, granulocyte-colony stimulating factor (G-CSF) has emerged as a potential treatment.5

Here, we report a case of natalizumab-associated GCN where diagnosis was aided by the novel use of FDG-PET, and immune reconstitution driven by G-CSF lead to disease stabilization and subsequent improvement.