The etiology of pericarditis has been discussed so often and by so many that it is indeed difficult to present a discussion of it to a body of this character in such a manner as to arouse interest. For this reason it seemed better to disregard the extensive literature upon this subject and utilize the reports of three hundred cases as a text, dwelling upon some points of interest presented by them and drawing attention to some things in which they differ from generally accepted statements. The cases in hand consist of 80 cases ending in recovery and 220 ending in death, of which 190 were examined postmortem; 244 of these cases are acute pericarditis and 56 are of the chronic form with partial or complete obliteration of the pericardial sac. The cases are derived in part from the records of the Cook County Hospital, where many of the cases