The Cancer Prevention Project 3 (CaPP3) will focus first on finding the right dose of aspirin for people with a mismatch repair gene defect, the underlying cause of Lynch syndrome. Three thousand people who have Lynch syndrome will be invited to take part in this study. On joining the study participants will be asked to provide a blood sample before receiving a supply of aspirin tablets. Participants will take three tablets each day for two years. One will be a dummy tablet and at least one will contain aspirin. We will keep a secret code so that no-one will know whether they are taking 100mg, 300mg or 600mg of aspirin per day. The blood samples will help us to investigate whether some people react differently to aspirin and also whether we can predict, using blood markers, who will develop cancers in the future. Anyone who takes part in CaPP3 will be invited to be part of a national registry which can provide careful follow-up to report side effects and provide detailed information about any cancers or polyps which are found on routine checkups.