• Myocardial infarction was followed in 10 instances by a febrile complication resembling idiopathic pericarditis with pleuritis and/or pneumonitis. A pericardial friction rub was heard in eight cases. Roentgenologic evidence of pericardial effusion was found in two cases. Pleural effusion occurred in seven. Associated with the signs of pericarditis was a peculiar temperature curve that showed protracted periods of low-grade fever between high peaks of fever and flare-ups of pain. Leukocytosis, occasionally giving counts as high as 35,000 cells per cubic millimeter, was present in 7 of the 10 cases. Frequent relapses prolonged the febrile course to weeks or months, but in all cases the outcome was favcrable.