Wilms tumor: A childhood form of kidney cancer with a peak age of occurrence at 3 years of age. It is sometimes associated with abnormalities of the urinary tracts or other birth defects. Some cases are related to defects in one of two genes referred to as Wilms' tumor 1 (WT1) or Wilms' tumor 2 (WT2). Symptoms can include abdominal pain, swelling, and blood in the urine. Diagnosis is made by biopsy, which can classify the tumors as having a favorable histology (microscopic appearance) or an unfavorable histology, which is associated with a worse outcome. The outcome is also reflected by the stage of the tumor (extent of spread) at the time of diagnosis. Treatment involves surgery and chemotherapy; sometimes radiation therapy is also recommended. Wilms tumor has a very high cure rate, particularly when detected as a localized tumor. Also known as nephroblastoma.