An occult intrapericardial yolk sac tumor occurred in a 3-year-old girl with a fatal outcome. At autopsy, a 5.5-cm mass surrounded the base of the heart and compressed the left atrium posteriorly. Histologically, the tumor was a pure yolk sac tumor. Postmortem chemical analyses of the blood revealed an alpha-fetoprotein level greater than 7000 microg/L. Acute myocarditis of both ventricles was also found.