Cerebrovascular disease and high blood pressure both appear to increase the risk of vascular dementia. PROGRESS aims to investigate whether blood pressure lowering with an angiotensin converting enzyme inhibitor-based regimen will reduce the risk of cognitive impairment in patients with a history of stroke or transient ischaemic attack. A total of at least 6000 patients will be randomised to receive perindopril (± indapamide) or matching placebo(s), with treatment and follow-up scheduled to continue for at least 4 years. Substudies will investigate the effects of treatment on cognitive decline in subgroups defined by apo-E genotype and on white matter lesions assessed by magnetic resonance imaging. Final results from the study should be available in 2001.