Opposing views have been published on the importance of ultrasound screening for abdominal aortic aneurysms. The Multicentre Aneurysm Screening Study was designed to assess whether or not such screening is beneficial. A population-based sample of men (n=67800) aged 65-74 years was enrolled, and each individual randomly allocated to either receive an invitation for an abdominal ultrasound scan (invited group, n=33839) or not (control group, n=33961). Men in whom abdominal aortic aneurysms (> or =3 cm in diameter) were detected were followed-up with repeat ultrasound scans for a mean of 4.1 years. Surgery was considered on specific criteria (diameter > or =5.5 cm, expansion > or =1 cm per year, symptoms). Mortality data were obtained from the Office of National Statistics, and an intention-to-treat analysis was based on cause of death. Quality of life was assessed with four standardised scales. The primary outcome measure was mortality related to abdominal aortic aneurysm. 27147 of 33839 (80%) men in the invited group accepted the invitation to screening, and 1333 aneurysms were detected. There were 65 aneurysm-related deaths (absolute risk 0.19%) in the invited group, and 113 (0.33%) in the control group (risk reduction 42%, 95% CI 22-58; p=0.0002), with a 53% reduction (95% CI 30-64) in those who attended screening. 30-day mortality was 6% (24 of 414) after elective surgery for an aneurysm, and 37% (30 of 81) after emergency surgery. Our results provide reliable evidence of benefit from screening for abdominal aortic aneurysms.