Wilms' tumor (WT) is the most common intra-abdominal malignancy found in children (0.8 per 100,000 people per year). It originates from the kidney by abnormal proliferation of the metanephric blastema. The histological spectrum of this tumor ranges from elements of the renal blastema to aggressive sarcomatous variants (4–10% of WT). An association has been described between the loss of the WT1 suppressor gene found in chromosome 11 and the development of this tumor, and it may be seen in relation to WAGR syndrome (W ilms' tumor