Men who watch more than four hours of TV a day significantly increase their risk of developing bowel cancer, researchers have found.
Experts tracked 500,000 British people for six years - and found a significant link between sedentary behavior and bowel cancer risk.
But the strongest risk was found among men, particularly those who watched lots of television.
The scientists, from the University of Oxford, Imperial College London and the UN International Agency for Research on Cancer, found men who sat in front of the television for more than four hours a day were 35 percent more likely to develop bowel cancer.
For women the increased risk of television watching was just 11 percent - but when they examined the figures they found this was not statistically significant.
Intriguingly, they found no link between computer screen time and bowel cancer risk.
The researchers suspect men are more likely to smoke, drink and eat junk food than women while watching TV.
The findings, published in the British Journal of Cancer, revealed 2,391 people of the 500,000 went on to develop bowel cancer.
Dr Neil Murphy, lead researcher based at IARC in France, said: 'Previous research suggests that watching TV may be associated with other behaviors, such as smoking, drinking and snacking more, and we know that these things can increase the risk of bowel cancer.
'Being sedentary is also associated with weight gain and greater body fat. Excess body fat may influence the blood levels of hormones and other chemicals which affect the way our cells grow, and can increase bowel cancer risk..'
Professor Linda Bauld, Cancer Research UK's prevention expert, said: 'This study poses interesting questions such as why screen time from computers didn't increase the risk of bowel cancer but watching TV did.
'There is evidence that greater exposure to TV junk food adverts increases the likelihood of eating more, which will also increase your chances of becoming overweight.
'It's interesting that only men who watched a lot of TV had an increased risk of bowel cancer, but not women.
'The study didn't look at this directly, but it could be because men might smoke, drink and eat more unhealthily than women while watching TV.
'We'll need further research to answer the questions this study raises. What we do know is that keeping a healthy weight, cutting back on alcohol, being physically active and eating a diet rich in fruit and vegetables are known to cut your risk of bowel cancer.'
But Professor Paul Pharoah of the University of Cambridge, said: 'If sedentary behavior were the explanation for the association with TV time, as speculated by the authors, then it would not depend on the cause of the sedentary behavior.
'There were some differences in the observed associations between men and women, but these differences were small and the most likely explanation is that they are chance findings.
'In short the implication of this study for individuals is nothing very new.
'We already know that eating a healthy diet, maintaining a healthy body weight, keeping active, limiting alcohol intake and not smoking reduces the risk of many diseases.'