In addition to longstanding evidence from observational studies, evidence from randomised trials of the effectiveness of aspirin for chemoprevention of colorectal cancer has increased substantially in recent years. Trials have shown that daily aspirin reduces the risk of any recurrent colorectal adenoma by 17 % and advanced adenoma by 28 %, and that daily aspirin for about 5 years reduces incidence and mortality due to colorectal cancer by 30-40 % after 20 years of follow-up, and reduces the 20-year risk of all-cause cancer mortality by about 20 %. Recent evidence also shows that the risk of major bleeding on aspirin diminishes with prolonged use, suggesting that the balance of risk and benefit favours the use of daily aspirin in primary prevention of colorectal and other cancers. Updated clinical guidelines are currently awaited.