Epidemiological studies have provided convincing evidence that obesity increases the risk for cancers of the oesophagus (adenocarcinoma), colon, pancreas, breast (post-menopausal), endometrium and kidney. The magnitude of the increase in risk varies between cancer sites. For an increase in BMI of 10 kg/m2 relative risks are approximately 2·3 for adenocarcinoma of the oesophagus, 1·5 for colon cancer in men, 1·2 for colon cancer in women, 1·4 for post-menopausal breast cancer, 2·9 for endometrial cancer and >1·5 for kidney cancer, while the size of the effect on cancer of the pancreas is uncertain. There is also evidence that obesity increases the risks for cancers of the gallbladder, malignant melanoma, ovary, thyroid, non-Hodgkin lymphoma, multiple myeloma and leukaemia. Estimates of the percentage of cancers that can be attributed to excess body weight suggest that in the UK and similar countries approximately 5% of all cancers are attributable to overweight and obesity.