Several earlier studies have found that, among patients with a history of colon polyps or cancer, regular aspirin treatment prevents the recurrence of precancerous polyps. However, the ability of aspirin to reduce the long-term incidence of invasive cancer has not been well-demonstrated. Our study did find a protective effect of long-term aspirin use on risk of invasive colorectal cancer, but only at dosage levels considerably higher than those used to prevent cardiovascular disease.