THE occurrence of pericarditis in association with rheumatic fever, myocardial infarction, tuberculosis, pneumonia, septicemia, generalized lupus and uremia is well known and usually creates no diagnostic problems. Acute pericarditis unassociated with these diseases has, however, been recognized.' These cases may be characterized by pain similar to that which occurs in acute myocardial infarction, necessitating the exercise of great care in differential diagnosis. This is of more than academic importance, since the treatment and prognosis of the two disorders are markedly different. During the past year 5 cases of this type of pericarditis were seen, and in 4 of them a mistaken diagnosis of acute myocardial infarction had been made. Those points that have been found most helpful in achieving a correct diagnosis will be discussed below.Case Reports Case 1. S. B., a 30-year-old, married man, was seen in another hospital on September 19, 1942. While convalescing from an attack of virus pneumonia, he was awakened at 1:30 a.m. on the day of admission by severe, constricting pain in the anterior chest and sharp precordial pain, markedly accentuated by bodily movements and inspiration. When he was examined within a few hours by his physician, a loud pericardial friction rub was heard. The pain was so severe and persistent that morphine had to be administered subcutaneously for relief.Physical examination on admission revealed a loud pericardial friction rub over the entire precordial area, diminished respiratory excursion of the left lower chest and a blood pressure of 142/76. The friction rub was constantly audible during the first 15 days. Daily swings in temperature up to 104°F. occurred for 12 days, and the pulse rate averaged 85. The white-cell count was usually normal, but on one occasion reached 14,300. Electrocardiograms were consistent with acute pericarditis and at no time showed the pattern of myocardial infarction. Several teleroentgenograms revealed considerable cardiac dilatation, with subsequent reversion to normal size, and small pleural effusions. Fluoroscopy showed normal ventricular pulsations.Sulfadiazine was administered in adequate doses both before and after admission, but was without benefit and was therefore discontinued. The patient rapidly improved and was discharged on the 32nd day.Case 2. M. D., a 29-year-old, unmarried woman, entered the Beth Israel Hospital on January 2, 1943. At the age of 9 or 10 she had had pain and swelling in all the joints, and since then stiffness and swelling of the fingers and ankles. She was nervous and unstable and occasionally noted coldness and blanching of the fingertips. There were no skin lesions or sensitivity to sunlight.The present illness began 4 days before admission with the gradual onset of dull, gnawing pain at the top of the left shoulder, becoming severe on motion or deep inspiration. Dyspnea developed and increased to such an extent that the patient was comfortable only when lying on several pillows. Two days before entry crushing pain in the pre...