Statistical studies of the frequency of the different etiologic types of acute pericarditis may be misleading because of an increasing recognition of nonspecific pericarditis. In our experience this form is the most common in private practice, while on the wards of a general hospital, tuberculous pericarditis is the most frequent variety. Despite improvements in diagnostic technics there still remains a significant number of patients with acute pericarditis, the etiology of which cannot be categorized accurately. This latter group will be narrowed only after further clinical description and laboratory study. The diagnosis of nonspecific pericarditis should be made carefully because of similarity of its onset to that of myocardial infarction and its benign course with tendency to relapse. Tuberculous pericarditis merits early recognition because of the improved prognosis following prompt treatment. Rheumatic pericarditis is often a problem in children and adolescents and indicates serious cardiac involvement; in adults the clinical course is generally more benign. Pericarditis occurring in uremia and myocardial infarction is usually an incidental finding in the course of a more important disease. Pyogenic pericarditis is now a rare occurrence except in certain infectious states. The treatment of acute pericarditis has been considerably improved by the introduction of the antibiotic drugs. The judicious use of pericardial paracentesis may be life-saving in certain instances or it may yield diagnostic information from which effective therapy can be instituted.