There is convincing evidence of a reduction in vascular disease by aspirin, and highly persuasive evidence of a reduction in cancer, in particular colorectal cancer. Aspirin also increases the risk of gastrointestinal and cerebral bleeding. However, overviews of randomised trial data suggests that gastrointestinal bleeding attributable to aspirin is not the most serious, because there is no evidence of any increase in deaths from bleeding. The involvement of inadequately treated hypertension in cerebral bleeding attributable to aspirin is uncertain. Aspirin is widely approved for vascular risk reduction but no regulatory body has yet approved its use for reduction of cancer risk. Nevertheless, the risk-benefit balance of aspirin should be evaluated in relation to both vascular disease and cancer reduction together. Aspirin prophylaxis would seem to be of particular value as a complement to colorectal cancer screening. © 2012 Springer Science+Business Media New York.