This is a report of an 11-year-old, prepubertal boy with acute-onset urinary urgency and frequency, obsessions and compulsions related to urination, severe mood lability, inattention, impulsivity, hyperactivity, and intermittent periods of immobilization. Fever, cough, otitis, and sinusitis preceded neuropsychiatric symptoms. Antistreptolysin O and DNAse B antibody titers were elevated, and magnetic resonance imaging revealed bilateral diffuse caudate nuclei swelling. Plasmapheresis resulted in significant and rapid clinical improvement of obsessive-compulsive disorder symptoms and a simultaneous decrease in basal ganglia swelling, consistent with an immune-mediated pathophysiological process involving group A β-hemolytic streptococci. Hyperactivity, impulsivity, and inattention improved with lorazepam, suggesting that the attention-deficit/hyperactivity disorder symptoms could be manifestations of catatonia.