Kidney cancer generally refers to renal cell cancer, which develops in the lining of the small tubes in the kidney. There is usually just a single tumour in one kidney, but sometimes there may be more than one tumour, or tumours in both kidneys. About 90% of kidney cancers are renal cell cancer, and of these, the most common subtype is clear cell renal cancer (named because the tumour cells appear very pale or clear when observed under a microscope). 

Apart from renal cell cancer, there are some other types of kidney cancers: 

• Transitional cell cancer of the renal pelvis and ureter starts in either the ureter (the long tube that connects the kidney to the bladder) or the renal pelvis (the top part of the ureter, where it connects to the kidney). The renal pelvis and ureters are lined with transitional cells, which can develop into cancer cells. These cancers behave more like bladder cancers than kidney cancers, so are treated in a similar way to bladder cancers. 
• Wilms tumours usually occur in children rather than adults; about 90% of kidney cancers in children are Wilms tumours. These tumours are often not detected until they are quite large, but most are found before they have spread (metastasised) to other organs. 
• Renal sarcomas, which are rare, begin in the blood vessels or other types of tissue in the kidney. 

Other types of tumours in the kidneys are benign – meaning they do not spread (metastasise) to other parts of the body.