Low-dose aspirin (acetylsalicylic acid) therapy has been shown to reduce the risk of vascular events and there is increasing evidence of its potential to reduce the rate of cognitive decline in the elderly. Adverse effects including gastrointestinal and intracranial haemorrhage may offset these benefits. The balance of risks versus benefits of aspirin for the primary prevention of cardiovascular disease and vascular dementia has not been established in the elderly. There is clearly a need to conduct a study in family practice to investigate whether routine use of low-dose aspirin for the primary prevention of cardiovascular disease and vascular dementia in the elderly is beneficial or harmful. Aspirin in reducing events in the elderly (ASPREE) is a placebo-controlled trial of low-dose aspirin for the primary prevention of major adverse cardiovascular events and vascular dementia. It will follow 15,000 subjects aged 70 years or more for an average of 5 years. This sample size has a power of 87% to detect a 15% reduction in primary events in the aspirin group, with an anticipated combined primary event rate of 20 per 1000 patient years.