BACKGROUND: Although endovascular aneurysm repair (EVAR) has a lower 30-day operative mortality than open repair, the long-term results of EVAR are uncertain. We instigated EVAR trial 1 to compare these two treatments in terms of mortality, durability, health-related quality of life (HRQL), and costs for patients with large abdominal aortic aneurysm (AAA). METHODS: We did a randomised controlled trial of 1082 patients aged 60 years or older who had aneurysms of at least 5.5 cm in diameter and who had been referred to one of 34 hospitals proficient in the EVAR technique. We assigned patients who were anatomically suitable for EVAR and fit for an open repair to EVAR (n=543) or open repair (n=539). Our primary endpoint was all-cause mortality, with secondary endpoints of aneurysm related mortality, HRQL, postoperative complications, and hospital costs. Analyses were by intention to treat. FINDINGS: 94% (1017 of 1082) of patients complied with their allocated treatment and 209 died by the end of follow-up on Dec 31, 2004 (53 of aneurysm-related causes). 4 years after randomisation, all-cause mortality was similar in the two groups (about 28%; hazard ratio 0.90, 95% CI 0.69-1.18, p=0.46), although there was a persistent reduction in aneurysm-related deaths in the EVAR group (4%vs 7%; 0.55, 0.31-0.96, p=0.04). The proportion of patients with postoperative complications within 4 years of randomisation was 41% in the EVAR group and 9% in the open repair group (4.9, 3.5-6.8, p>0.0001). After 12 months there was negligible difference in HRQL between the two groups. The mean hospital costs per patient up to 4 years were UK pound sterling 13,257 for the EVAR group versus pound sterling 9946 for the open repair group (mean difference pound sterling 3311, SE 690). INTERPRETATION: Compared with open repair, EVAR offers no advantage with respect to all-cause mortality and HRQL, is more expensive, and leads to a greater number of complications and reinterventions. However, it does result in a 3% better aneurysm-related survival. The continuing need for interventions mandates ongoing surveillance and longer follow-up of EVAR for detailed cost-effectiveness assessment.