Obesity increases the risk of cancers of the bowel, breast, endometrium, kidney, pancreas, liver and gallbladder. It also increases the risk of aggressive prostate cancer and of some oesophageal cancers and stomach cancers, and there is some evidence of associations with other cancers as well. About 3-8% of cancers in Australia can be attributed to obesity. Weight gain during adult life is also associated with increased risk of various cancers. The relationship between obesity and risk of cancer has largely been established from cohort studies in which few participants were overweight or obese during early life. For example, in the Melbourne Collaborative Cohort Study (MCCS), 17% of males and 10% of females reported being overweight or obese as young adults, whereas in the most recent Australian Health Survey, 41% of males aged 18-24 and 32% of females were overweight or obese. MCCS participants gained substantial weight during adult life – at study entry, when they were aged 40-69, 72% of males and 57% of females were overweight or obese. What effect early life obesity will have on future cancer rates is largely unknown, although evidence regarding other obesity-related non-communicable diseases suggests that it will further increase the burden.