A 12-year-old boy with necrotizing arteritis developed massive pericardial effusion and cardiac tamponade. Eosinophilia of the blood and the pericardial fluid was striking. A pericardiectomy established the diagnosis of allergic granulomatous angiitis of Churg and Strauss. The pericarditis resolved, but it was followed by severe myocardial involvement, resulting in chronic congestive heart failure. The patient was treated with anticongestive, antiinflammatory, and immunosuppressive agents, but died 28 months after the onset of cardiac symptoms.