Renal tumors are the fifth most common cancer in children under 15 years of age and it is estimated that 7.1 per million children are affected, and 500 new cases are reported each year in the United States (Gurney et al. 1995). Wilms tumor is found more often in black than in white children, with a ratio of 1.25:1. Wilms tumor is slightly less frequent in boys than in girls. For unilateral tumors the median age at diagnosis is 41.5 months for boys and 46.9 months for girls. For bilateral tumors, the median age at presentation is 29.5 months for boys and 32.6 months for girls (Breslow et al. 1993).