The mediastinum is a common site for germ cell tumors. We describe an unusual case in an 18-month-old girl who presented with respiratory distress and cardiomegaly. An echocardiogram revealed a large pericardial effusion and a mass adjacent to the heart. She underwent detailed investigation and at open exploration was found to have a wholly intrapericardial malignant yolk sac tumor. Staging investigations revealed a bony metastasis. The serum marker alpha-fetoprotein was elevated. She was treated with chemotherapy and is in remission 1 year after completion of treatment. We review the methods of diagnosis and follow-up of children with these tumors.