It seems evident that low doses (75 mg/day) of aspirin are adequate for attempts to prevent thrombotic occlusion of blood vessels. There are no data to suggest that different doses should be used based on whether primary or secondary prophylaxis is planned. However, even low doses of aspirin are associated with relatively small numbers of serious haemorrhagic complications. These complications are outweighed by the benefits of therapy in many examples of secondary prevention in high-risk groups that have already experienced an event or symptoms because of underlying vascular disease. Primary prophylaxis is a more controversial area. There is generally less to gain, and more to lose, when measures are applied to large numbers of asymptomatic people. Based on the available data, the conservative approach for aspirin use will be to reserve primary prophylaxis for those people with the highest estimated risk.