In a study of the time of appearance of mural thrombus and the relation between thrombosis and congestive heart failure in acute viral myocarditis 180 BALB/c mice were inoculated intraperitoneally with the M variant of encephalomyocarditis virus. Thirty mice were killed on day 5 and 50 mice each on days 7, 10, and 14. Of the 176 mice with myocarditis, 25 (14.2%) developed congestive heart failure after day 9 and 22 (12.5%) showed thrombi after day 6. Most of the thrombi were at the sites of endocardial lesions in the atria. The total incidence of thrombosis in mice with congestive heart failure was significantly higher than that in mice without. None of the 50 uninoculated control mice developed thrombi. The present study suggests that acute viral myocarditis carries an appreciable risk of thromboembolism and that congestive heart failure is a risk factor for thrombosis.