Thirteen patients with tuberculous pericarditis (12 men and 1 woman aged 13 to 70 years [mean 41]) were identified in a group of 294 patients consecutively admitted for primary acute pericardial disease. The diagnosis was made by the following studies: sputum culture (n = 4), culture of pericardial fluid obtained by pericardiocentesis (n = 3), histologic study and culture of pericardial biopsy (n = 3), lymph node biopsy (n = 2) and pleural biopsy (n = 1). Clinical presentation was remarkably variable: four patients had an acute, apparently self-limited course, one had relapsing tamponade, four had tamponade effectively treated with pericardiocentesis and four had toxic symptoms with persistent fever. The interval from hospital admission to diagnosis ranged from 1 to 14 weeks (mean 5.2). Constrictive pericarditis developed in six patients and effusive-constrictive pericarditis in one; all seven required pericardiectomy 2 to 3.5 months after admission. No patient died. It is concluded that 1) tuberculous pericarditis has a variable clinical presentation and therefore it should be considered in the evaluation of all instances of pericarditis without a rapidly self-limited course; 2) the diagnosis should be based only on objective data obtained with a systematic study protocol; 3) early definitive diagnosis is still difficult to achieve; and 4) development of subacute constrictive pericarditis requiring pericardiectomy is common.