Obesity is fuelling a major increase in the number of cases of kidney cancers diagnosed in Britain, experts say.
Cancer Research UK has published figures showing there were just over 9,000 cases in 2009, compared with just under 2,300 in 1975.
Obesity increases kidney cancer risk by about 70%, compared with smoking which increases it by about 50%.
Cancer Research UK says too few people understand the cancer risk of being overweight.
Kidney cancer is now the eighth most common cancer.
Blood in the urine can be an early sign that something is wrong.
But kidney cancer, if diagnosed early, can often be cured by surgery.
The experts say that being overweight increases the risk of this cancer, as well as others including breast, bowel and womb cancer, because it causes the higher levels of certain hormones to be produced, compared with those seen in healthy people.
Smoking rates in the UK have fallen over the last 35 years, but overweight and obesity levels are rising – with nearly 70% of men and almost 60% of women in the UK having a BMI of 25 or more – which means they are classed as being overweight.
However the five-year survival rate for kidney cancer has increased by 85% between the early 1970s and 2005-09.
Professor Tim Eisen, a Cancer Research UK kidney cancer expert based at the University of Cambridge, said: “Over the last 10 years, Cancer Research UK has helped to develop new drugs which destroy the blood supply to the kidney cancers. These drugs control the disease in most patients but do not cure it.
“It is best to prevent the problem in the first place – maintaining a healthy weight and not smoking are the best ways of doing that.”
Sara Hiom, director of information at Cancer Research UK, said: “Too few people know about the significant cancer risks associated with being very overweight.
“While giving up smoking remains the best way to cut your chances of developing kidney cancer, the importance of keeping a healthy weight shouldn’t be overlooked.”