Dietary habits, physical activity and body fatness influence our risk of cancer, especially colorectal cancer. To find out whether this is also true for individuals with the inherited Lynch syndrome (LS), which puts them at a very high risk of (colorectal) cancer at a young age, the research in this thesis was conducted. The findings show that a higher body mass index at young adulthood also increases the risk of colorectal cancer, and other types of cancer, among women with LS. Although results show that the cancer prevention recommendations to eat a healthy diet and maintain a healthy body weight – also at young adulthood – may also be beneficial to those with a very high cancer risk due to LS, we did not see that persons with LS change their lifestyle after a colorectal tumour diagnosis. More attention to lifestyle is suggested for those involved in the care of individuals with LS.