Wilms tumor is the most common kidney cancer in children. In Europe and North America, Wilms tumor affects 1 in 10,000 children. In the United States, 500 children develop Wilms tumor each year. The incidence of Wilms tumor seems to vary among populations, with African Americans having a higher-than-average risk of developing this cancer and Asians having a lower-than-average risk.

Wilms tumor rarely develops in adults; only about 300 such cases have been described.

Changes in any of several genes are involved in the formation of Wilms tumor. Wilms tumor is often associated with mutations in the WT1 gene, CTNNB1 gene, or AMER1 gene. These genes provide instructions for making proteins that regulate gene activity and promote the growth and division (proliferation) of cells. WT1, CTNNB1, and AMER1 gene mutations all lead to the unchecked proliferation of cells, allowing tumor development.

Changes on the short (p) arm of chromosome 11 are also associated with developing Wilms tumor. Two genes in this area, IGF2 and H19, are either turned on or off depending on whether the copy of the gene was inherited from the mother or the father. This parent-specific difference in gene activation is a phenomenon called genomic imprinting. In some cases of Wilms tumor, abnormalities in the process of genomic imprinting on chromosome 11 lead to a loss of H19 gene activity and increased activity of the IGF2 gene in kidney cells. The resulting loss of H19 gene activity, which normally restrains cell growth, and increase in IGF2 gene activity, which promotes cell growth, together lead to uncontrolled cell growth and tumor development in people with Wilms tumor.

In most cases of Wilms tumors involving one kidney and nearly all cases involving both kidneys, the tumors are thought to arise from immature kidney tissue that never developed properly. These immature tissues are known as nephrogenic rests. It is likely that genetic changes are involved in the presence of nephrogenic rests and that additional genetic changes trigger nephrogenic rests to develop into a tumor.

Genetic conditions that share a genetic cause with Wilms tumor can also have this cancer as a feature. These conditions include WAGR syndrome, Denys-Drash syndrome, and Frasier syndrome, which are caused by mutations in the WT1 gene. Wilms tumor has also been seen in individuals with Beckwith-Wiedemann syndrome, which can be caused by changes in the genomic imprinting of the IGF2 and H19 genes. Wilms tumor can be a feature of other genetic conditions caused by mutations in other genes.

Many children with Wilms tumor do not have identified mutations in any of the known genes. In these cases, the cause of the condition is unknown. It is likely that other, unknown genes are also associated with the development of Wilms tumor.