Most cancer cases are down to bad luck rather than unhealthy lifestyles, diet or even inherited genes, according to new research.
A US study found random mutations in DNA when cells divide are responsible for two thirds of cancers.
The remaining third are linked to environmental factors or defective inherited genes.
But the scientists warned a poor lifestyle could add to the 'bad luck factor'.
They found the more often cells divide, the more likely it is that letters of their genetic code will become jumbled, leading to an increased cancer risk.
"Bad luck" mutations that occur when one chemical letter in DNA is wrongly swapped for another during cell replication largely explained 22 of the 31 cancer types studied.
The remaining nine had incidence rates higher than predicted by bad luck, presumably due to the influence of environmental or inherited factors.
Overall, the study found that random mutations due to stem cell division could explain around 65% of cancer incidence.
Professor Bert Vogelstein, from Johns Hopkins University School of Medicine in the US, said: "All cancers are caused by a combination of bad luck, the environment and heredity, and we've created a model that may help quantify how much of these three factors contribute to cancer development, Prof Vogelstein said.
"This study shows that you can add to your risk of getting cancers by smoking or other poor lifestyle factors.
"However, many forms of cancer are due largely to the bad luck of acquiring a mutation in a cancer driver gene regardless of lifestyle and heredity factors.
"The best way to eradicate these cancers will be through early detection, when they are still curable by surgery."