Wilms' tumor or Wilms tumor, also known as nephroblastoma, is a cancer of the kidneys that typically occurs in children (rarely in adults), and occurs most commonly as a renal tumor in child patients. It is named after Max Wilms, the German surgeon (1867–1918) who first described it.

Approximately 650 cases are diagnosed in the U.S. annually. The majority of cases occur in children with no associated genetic syndromes; however, a minority of children with Wilms' tumor have a congenital abnormality.  It is highly responsive to treatment, with about 90 percent of children being cured.

Typical signs and symptoms of Wilms' tumor include the following:^{[citation needed]}

Wilms' tumor has many causes, which can broadly be categorized as syndromic and non-syndromic. Syndromic causes of Wilms' tumor occur as a result of alterations to genes such as the Wilms Tumor 1 (WT1) or Wilms Tumor 2 (WT2) genes, and the tumor presents with a group of other signs and symptoms. Non-syndromic Wilms' tumor is not associated with other symptoms or pathologies. Many, but not all, cases of Wilms' tumor develop from nephrogenic rests, which are fragments of tissue in or around the kidney that develop before birth and become cancerous after birth. In particular, cases of bilateral Wilms' tumor, as well as cases of Wilms' tumor derived from certain genetic syndromes such as Denys-Drash syndrome, are strongly associated with nephrogenic rests. Most nephroblastomas are on one side of the body only and are found on both sides in less than 5% of cases, although people with Denys-Drash syndrome mostly have bilateral or multiple tumors. They tend to be encapsulated and vascularized tumors that do not cross the midline of the abdomen. In cases of metastasis it is usually to the lung. A rupture of Wilms' tumor puts the patient at risk of bleeding and peritoneal dissemination of the tumor. In such cases, surgical intervention by a surgeon who is experienced in the removal of such a fragile tumor is imperative.^{[citation needed]}

Pathologically, a triphasic nephroblastoma comprises three elements:

Wilms' tumor is a malignant tumor containing metanephric blastema, stromal and epithelial derivatives. Characteristic is the presence of abortive tubules and glomeruli surrounded by a spindled cell stroma. The stroma may include striated muscle, cartilage, bone, fat tissue, and fibrous tissue. Dysfunction is caused when the tumor compresses the normal kidney parenchyma.^{[citation needed]}

The mesenchymal component may include cells showing rhabdomyoid differentiation or malignancy (rhabdomyosarcomatous Wilms).^{[citation needed]}

Wilms' tumors may be separated into two prognostic groups based on pathologic characteristics:^{[citation needed]}