Ethnic background: The risk of Wilms' tumor is higher in African-Americans than in Caucasians. Asian-Americans have the lowest risk of developing the disease.

Genetics: Most cases of Wilms' tumor are non-hereditary. However, some children appear to develop Wilms' tumor for genetic reasons. A small number of patients have a relative with the disease. Children with Wilms' tumor may have inherited an abnormal gene from a parent that increases the risk of this cancer.

An altered, damaged or missing gene may also cause Wilms' tumor. Such gene mutations can also cause other birth defects. About 15 percent of children with Wilms' tumor may have other birth defects.

WAGR syndrome is a birth defect that includes Wilms' tumor, a lack of the colored area in the child's eyes (aniridia), defects of the kidneys, urinary tract, clitoris, ovaries, penis, scrotom or testicles (genitorurinary tract abnormalities) and mental retardation. Children with WAGR syndrome have a 33 percent chance of developing Wilms' tumor.

Children with Beckwith-Wiedemann syndrome have a high risk of developing Wilms' tumor. This syndrome is characterized by having larger-than-normal internal organs and an oversized leg or arm on one side of the body. Children with this syndrome may experience kidney cysts and tumors of the pancreas or adrenal gland.

Wilms' tumor may also occur in boys with Denys-Drash syndrome, in which the penis, testicles and scrotum do not develop.