Two children who were brought to the Istanbul Children's Hospital with congestive heart failure caused by extension of Wilms' tumor to the right atrium are presented. In both cases a large mass was noted in the right atrium by two-dimensional echocardiography. The tumors were successfully removed at open heart surgery, and chemotherapy and radiotherapy were started postoperatively. The patients are both alive and symptom-free; one, three and a half years and the other two years postoperatively.