We reported a 10-year-old boy with an interhemispheric subdural empyema caused by acute excerbation of frontal sinusitis. He has suffered from allergic rhinitis and pansinusitis for years. He had fever and a generalized tonic-conic seizure, followed by prolonged consciousness disturbance and right hemiparesis. CT and MRI were useful to diagnose an interhemispheric subdural empyema, which showed a characteristic double-layer fluid collection pattern. Prompt diagnosis and treatment with craniotomy resulted in recovery without any neurological sequelae. His basic disorders, allergic rhinitis and pansinusitis, were successfully treated with macrolides and antiallergic drugs: pranlukast hydrate and sodium cromoglicate, administered per os and by inhalation, respectively.