This paper evaluates aspirin in the reduction of colorectal cancer risk against the nine causality criteria suggested by Bradford-Hill in 1965. Although some questions remain, the evidence is suggestive of a reduction by perhaps 20-30%. Aspirin could make important contributions to public health programmes given that it reduces cardiovascular disease risk and is relatively safe. It is appropriate for bodies such as the World Health Organisation and national governments to begin to consider the future use of aspirin for the reduction of two major sources of death and disability.