The clinical and radiographic appearances of four children with cardiac extension of Wilms' tumor and four cases from the literature are described. Four of the eight children were seen for "cardiac problems" and four for "routine" Wilms' tumor. In those "routine" cases, there were no clinical suggestions of inferior vena cava or cardiac extension. Preoperative screening for tumor extension may be crucial. However, because of the rarity of cardiac extension, it would be appropriate to screen patients by noninvasive methods such as gray scale ultrasound of the abdomen, echocardiography, or computed tomography before any invasive procedure is considered.