In 16 of 177 patients with effusive acute idiopathic pericarditis (10 men, 6 women, mean age 38 years), features of cardiac constriction were detected (by physical examination in 6 patients and by noninvasive recordings in all) between 5 and 30 days after an echocardiogram had shown pericardial effusion, at a time when signs of activity had abated and effusion was already minimal or had altogether disappeared. Cardiac catheterization was performed in 5 patients, showing either overt (3 patients) or occult (2 patients) cardiac constriction. Two patients had clinical signs of cardiac failure. After a mean of 2.7 months, the features of constriction had spontaneously disappeared in all patients in the clinical examination and noninvasive recordings, and remained so in subsequent control studies (mean follow-up 31 months). Repeat cardiac catheterization in the 5 patients in whom it had been previously performed showed normal features both in the basal state and after fluid overload. The results of the present study show that some patients may go through a transient phase of cardiac constriction at the end of the effusive period of acute idiopathic pericarditis. Features of constriction are, in most cases, subtle and can go unrecognized if not specifically sought. However, they may have clinical relevance in some patients. These findings provide insight into the resolution phase of effusive acute idiopathic pericarditis, and an unnecessary pericardiectomy may be avoided.