Cancer of the kidneys, also known as renal cancer, is often curable when detected early. However, some patients may not be diagnosed until the tumor is large or has metastasized (spread to other organs) and becomes difficult to treat. Most kidney cancers are renal cell carcinomas. In children, the most common type of kidney cancer is called Wilms tumor. This Patient Page is based on one published in the July 7, 2004, issue of JAMA.