Five weeks after commencing highly active antiretroviral therapy, a 12-year-old boy with advanced human immunodeficiency virus infection presented with acute cerebellar dysfunction and hemiparesis. Progressive multifocal leukoencephalopathy was diagnosed by cerebrospinal fluid polymerase chain reaction for JC virus and magnetic resonance imaging of the brain. Rapid and sustained improvement followed a prolonged course of glucocorticosteroid therapy while continuing antiretrovirals.