Inclusion Criteria:
* Underwent screening or surveillance colonoscopy at MGH within the last 9 months with removal of at least one adenoma.
* Age 18-80 years.
* This study will only include adult participants because colorectal carcinogenesis in children is more likely to be related to a cancer predisposition syndrome with distinct biological mechanisms compared with sporadic colorectal cancer in adults. Patients over age 80 will not be enrolled since the benefits and risks of a daily aspirin regimen over the age of 80 have not yet been well-characterized.
* ECOG performance status ≤2 (Karnofsky ≥60%, see Appendix A)
* Not currently taking aspirin (any dose) within the last 6 months.
* The effects of aspirin on the developing human fetus are unknown. For this reason, women of child-bearing potential must agree to use adequate contraception (hormonal or barrier method of birth control; abstinence) prior to study entry and for the duration of study participation. Should a woman become pregnant or suspect she is pregnant while she is participating in this study, she should inform her treating physician immediately.
* Ability to understand and the willingness to sign a written informed consent document.
Exclusion Criteria:
* Use of any non-aspirin non-steroidal anti-inflammatory drug (NSAID) at any dose at least three times a week during the two months prior to randomization.
* Diagnosis of inflammatory bowel disease, liver or kidney disease, bleeding diathesis
* Any prior diagnosis of gastrointestinal cancer (including esophageal, small intestine, colon, pancreatic), or any diagnosis of other cancers (with the exception of non- melanoma skin) in which there has been any active treatment within the last three years
* Participants who are receiving any other investigational agents.
* History of allergic reactions attributed to compounds of similar chemical or biologic composition to aspirin.
* Known diagnosis of Familial Adenomatous Polyposis (FAP) or Hereditary Non-Polyposis Colorectal Cancer (HNPCC, Lynch Syndrome).
* Any adenoma that was not completely removed during previous colonoscopy.
* History of aspirin intolerance, bleeding diathesis, peptic ulcer or gastrointestinal bleed, endoscopic complications, or contraindication to colonoscopy.
* Inability or unwillingness to abstain from non-protocol use of aspirin or NSAIDs or to provide blood, urine, or stool samples or colon biopsies during the study.
* Uncontrolled intercurrent illness including, but not limited to, ongoing or active infection, symptomatic congestive heart failure, unstable angina pectoris, cardiac arrhythmia, or psychiatric illness/social situations that would limit compliance with study requirements.
* Pregnant or breastfeeding.
* Pregnant women are excluded from this study because aspirin is an FDA Category D agent with the potential for teratogenic or abortifacient effects. Because there is an unknown but potential risk for adverse events in nursing infants secondary to treatment of the mother with aspirin, breastfeeding should be discontinued if the mother is treated with aspirin.
* Participant must be able to swallow pills.
* Participant is taking any anticoagulant agent (e.g. warfarin) or antiplatelet agent (e.g. clopidogrel).
