A 48-year-old woman was referred with an 18-year history of focal-onset seizures. She also reported years-long slowly progressive right-sided weakness that was corroborated on examination. Repeated brain MRIs over 15 years showed multifocal left hemispheric T2 fluid-attenuated inversion recovery-hyperintense lesions with patchy enhancement and microhemorrhages, no diffusion restriction, and a left cerebellar infarct (Figure 1, A-F). Only 2 nonspecific white matter lesions were seen contralaterally, indicating largely unihemispheric disease. Differential diagnosis included unilateral primary angiitis of the CNS (PACNS), Rasmussen encephalitis, and myelin oligodendrocyte glycoprotein antibody-associated disease.1 Serum and CSF testing for autoimmune, infectious, and malignant etiologies and whole-body fluorodeoxyglucose-PET, whole-exome genetic sequencing, and MR vessel-wall imaging were nondiagnostic. Brain biopsy revealed vasculitis (Figure 2, A-F), and the patient was diagnosed with unilateral PACNS. Treatment with mycophenolate mofetil has been initiated. Unilateral PACNS is a rare unihemispheric disease characterized by an indolent course and seizures, recognition of which is critical to accurate diagnosis.1,2.