OBJECTIVE To investigate the routine use of low dose aspirin in people aged > or = 70 without overt cardiovascular disease. DESIGN Epidemiological modelling in a hypothetical population. SETTING Reference populations of men and women in the year 2000 from the state of Victoria, Australia. SUBJECTS 10,000 men and 10,000 women aged 70-74 with no cardiovascular disease. MAIN OUTCOME MEASURES First ever myocardial infarction or unstable angina, ischaemic or haemorrhagic stroke, and major gastrointestinal haemorrhage. Health adjusted years of life lived. RESULTS The proportional benefit gained from the use of low dose aspirin by the prevention of myocardial infarctions (-389 in men, -321 in women) and ischaemic stroke (-19 in men and -35 in women) is offset by excess gastrointestinal (499 in men, 572 in women) and intracranial (76 in men, 54 in women) bleeding. The results in health adjusted years of life lived (which take into account length and quality of life) are equivocal for aspirin causing net harm or net benefit. CONCLUSION Epidemiological modelling suggests that any benefits of low dose aspirin on risk of cardiovascular disease in people aged > or = 70 are offset by adverse events. These findings are tempered by wide confidence intervals, indicating that the overall outcome could be beneficial or adverse.