Primary idiopathic pericarditis has been recognized as a clinical entity for many years. It has long been suspected that viral infection may produce this condition. Only in recent years, however, have specific viruses been identified in the pericardial fluid of such patients.1This is the second report of the isolation of influenza virus from the pericardial fluid of an individual with pericarditis.2 Report of a Case A 5-year-old girl was admitted to the St. Joseph Mercy Hospital because of a febrile illness with cardiac tamponade. She had been well until 7 days prior to admission when she developed intermittent fever, malaise, and anorexia. For the 4 days prior to admission she stayed in bed voluntarily and one day before entry developed tachypnea and pain in the right shoulder. She had no cough, other joint pain, or epistaxis. The temperature was 102 F, the pulse 152 per minute, and