Smoking while underage - or even being exposed to second-hand smoke as a child - increases the risk of bladder cancer in later life, a study suggests.
Breathing in smoke could increase risk, the study suggests
Researchers funded by Cancer Research UK looked at data on almost 430,000 people across Europe.
They found people who smoked before the age of 15 were three times more likely to get bladder cancer later in life.
Those exposed to second-hand smoke in childhood were almost 40% more likely to develop bladder cancer.
The disease is the fourth most common cancer among men and kills more than 4,800 people in the UK each year.
The research was published in the International Journal of Cancer.
The researchers analysed data from the 429,906 people taking part in the European Prospective Investigation of Cancer and Nutrition (EPIC), 633 of whom went on to develop bladder cancer during the following six years.
They also looked at the smoking habits and exposure to smoke.
Being exposed to parental smoking, or second-hand smoke in another way, as a child was linked to an increased risk of the cancer - although exposure as an adult at home or at work was not.
The finding is in line with previous studies.
Overall smokers were four times more likely to develop bladder cancer. But both the intensity and duration of smoking affected their risk.
People who had started smoking before they were 15 had a three-fold higher risk of bladder cancer than those who had never smoked.
If someone started smoking between the ages of 15 and 19 they had a 1.5-fold higher risk.
Writing in the International Journal of Cancer, the researchers said: "The indication in our study is that early exposure to tobacco smoke might increase the risk of bladder cancer calls for further research and adds to the body of evidence suggesting that children are more sensitive to carcinogens [cancer-causing agents] than adults."
Dr Naomi Allen of Oxford University, a UK researcher who worked on the study, said: "Previous research has shown that there's a strong link between smoking and bladder cancer.
"But this study also suggests that young people who are exposed to second-hand smoke are more at risk of going on to develop the disease in later life.
"This adds to the growing body of evidence that children and adolescents may be even more vulnerable to the harmful effects of tobacco smoke than adults."
Professor John Toy, Cancer Research UK's medical director, said: "Although more research is needed to confirm the seeming effects of childhood exposure to second-hand smoke, the study's findings support the health value of the smoking ban in public places."