Essential for life, your kidneys are two bean-shaped organs near the middle of your back. They cleanse your blood and maintain levels of important chemicals such as sodium and potassium. They also help regulate your blood pressure. Blood travels through the renal arteries to your kidneys, which filter out waste products and extra water and convert them into urine. Urine leaves your kidneys through two tubes (ureters) and flows to your bladder for storage until you urinate. Urine leaves your bladder through a narrow tube called the urethra.

In Wilms' tumor, doctors believe the cancer arises when cells destined to form the kidney fail to develop and instead multiply in their primitive state. The tumor usually becomes evident between ages 1 and 5. The mass often distorts the kidney and can compress normal tissue into a thin rim. It may cause bleeding and can result in blood leaking into the urine.

Most cancers occur after damage to genes, sets of instructions within cells that direct the manufacture of every structure of the body. Genes program cells to develop, grow, mature and die. Cancer results when changes (mutations) arise in genes that control growth, allowing cells to multiply without restraint. In rare cases, genetic defects pass from parent to child. This occurs in 1 percent to 2 percent of children with Wilms' tumor.

• cancer

Wilms' tumor occurs more frequently in children with certain abnormalities present at birth:

• Aniridia. In this rare condition the iris — the membrane that gives the eye its color and controls the amount of light entering the eye — fails to form completely.

• Hemihypertrophy. This abnormality occurs when one side of the body is noticeably larger than the other side.

Other abnormalities are related to the genitals and urinary system:

• Cryptorchidism. One or both testicles fail to descend into the scrotum.

• Hypospadias. The urinary (urethral) opening is not in its normal position at the tip of the penis, but is on the underside.

Wilms' tumor can occur as part of rare syndromes, including:

• WAGR syndrome. This syndrome includes Wilms' tumor, aniridia, abnormalities of the genitals and urinary system, and mental retardation.

• Denys-Drash syndrome. This syndrome includes Wilms' tumor, kidney disease and male pseudohermaphroditism, in which a male is born with undescended testicles.

• Beckwith-Wiedemann syndrome. Included in this syndrome are: omphalocele, which occurs when the abdomen fails to close around the base of the umbilical cord and abdominal organs protrude into the base of the cord; a large tongue (macroglossia); and enlarged internal organs.

Take your child to the doctor if you detect an abdominal mass or if you notice blood in his or her urine. Other signs and symptoms — fever, high blood pressure, constipation, reduced appetite, abdominal pain and weight loss — may not on their own be associated with Wilms' tumor, but they merit a trip to the doctor for evaluation.

• Abdominal pain

• cancer

• constipation

• fever

• High blood pressure

If your child has been treated for Wilms' tumor, report any new signs or symptoms to your child's doctor. These problems may be a sign of cancer returning or of side effects of treatment.