Too much exercise could be bad for cancer patients, according to a study.
Scientists found that prostate tumours in mice grew more quickly when the animals were active.
They speculate that exercise might increase the blood flow to tumours, thereby assisting their growth. However, the researchers caution that human patients may not respond the same way as animals.
In the US study, prostate tumours were transplanted into the flanks of 50 mice. Half the animals were then placed in cages with exercise wheels, and all were fed the same diet.
On average, the mice with access to exercise wheels ran more than half a mile each day.
Dr Lee Jones, from Duke University in Durham, North Carolina, said: "We found that among the mice that had the opportunity to voluntarily exercise, tumours grew approximately twice as fast as they did among the mice that did not have the opportunity to exercise."
The findings were presented at the annual meeting of the American Association for Cancer Research in San Diego, California.
Tumours need a good blood supply to thrive, and send out signals which stimulate the building of new blood vessels - a process known as angiogenesis.
Researchers are actively looking at ways to combat this process to tackle cancer.