Eight cases of acute benign “idiopathic” or “nonspecific” pericarditis are reported in which an influenza virus was associated by history and serologic evidence with the acute pericarditis. The laboratory evidence demonstrated in five cases the type B (Great Lakes) influenza virus; in two cases both type A (Japanese) and type B viruses; and in one case the type A virus. Seven of the eight patients reported were males. There was an interval of one or more weeks between the acute episode of “influenza” or “upper respiratory infection” and the cardiac disorder. Acute “nonspecific” pericarditis is not necessarily “benign,” as it is frequently associated with myocarditis, pericardial effusion, pleurisy, pleural effusion, and pneumonia. These features may be more appropriately designated as acute myopericarditis. All patients with “postpartal heart disease,” atypical myocardial infarction, or acute “nonspecific” pericarditis should be carefully evaluated for the recent presence of viral diseases as a possible cause of the cardiac difficulty. The hemagglutination inhibition influenzal antibody titer determinations, an ancillary service of the state laboratory, are recommended in all cases of acute “nonspecific” pericarditis in an effort to further classify these cases. The history and the clinical and laboratory findings in these eight cases of “nonspecific” pericarditis suggest that this cardiac disorder is in fact a myopericarditis and the term postviral myopericarditis is suggested in preference to acute benign “nonspecific” or “idiopathic” pericarditis.