Wilms tumor is the most common type of childhood kidney cancer. It's usually diagnosed in children under 5 but can also affect older kids. With early diagnosis and treatment, patients with Wilms tumor usually have a good prognosis. The great majority can be cured.

Most of the time, the cause of Wilms tumor is unknown. But in about 10% of cases, the condition may result from specific genetic mutations or changes in the activity of certain genes. It's more common in children who have birth defects affecting the urinary system or genitals and in those who are missing an iris (the colored part of the eye) or who have hemihypertrophy (when one side of the body grows larger than the other). There's also increased risk of Wilms tumor among children with certain congenital syndromes, including Beckwith-Wiedemann, Denys-Drash, Frasier and WAGR syndromes.

Although it's less common after childhood, Wilms tumor can affect older adolescents and even adults. Adults with Wilms tumor may discover they have it during treatment for another type of kidney cancer. While the prognosis isn't as positive as for young children, the survival rate for patients ages 15 to 39 is 75%.

Treatment for early-stage Wilms tumor is usually successful. When the cancer hasn't spread, the survival rate is 90%.