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Background: Abdominal aortic aneurysm (AAA) rupture is a common cause of sudden death. Pre-emptive elective surgical repair can prevent aneurysm rupture and be life-saving. Ultrasmall superparamagnetic particles of iron oxide (USPIO) detect cellular inflammation using magnetic resonance imaging (MRI) in patients with AAAs. For this reason, USPIO-enhanced MRI represents a promising new technique that could improve risk prediction and better guide surgical intervention. Objectives: To assess whether or not USPIO-enhanced MRI can predict aneurysm growth rates and clinical outcomes in patients with AAAs. Design: A prospective multicentre open-label observational cohort study. Setting: Three secondary and tertiary care hospitals in Scotland. Participants: Patients (n = 342) aged > 40 years with a maximum anteroposterior AAA diameter of ≥ 40 mm confirmed by abdominal ultrasonography, and under ultrasonographic surveillance as part of routine clinical care. Interventions: USPIO-enhanced MRI of AAA. Main outcome measures: The primary end point was the composite of aneurysm rupture or repair. Secondary outcomes included rate of aneurysm growth, all-cause mortality and aneurysm-related mortality. Results: Participants (85% male, aged 73.1 ± 7.2 years) had a baseline aneurysm diameter of 49.6 ± 7.7 mm, and USPIO enhancement was identified in 146 participants (42.7%), absent in 191 participants (55.8%) and indeterminate in 5 participants (1.5%). During follow-up (1005 ± 280 days), there were 17 AAA ruptures (5.0%), 126 AAA repairs (36.8%) and 48 deaths (14.0%). Compared with those without uptake, patients with USPIO enhancement have increased rates of aneurysm expansion (3.1 ± 2.5 vs. 2.5 ± 2.4 mm/year; p = 0.0424), although this was not independent of current smoking habits (p = 0.1993). The primary end point (aneurysm rupture or repair) occurred more frequently in participants with USPIO enhancement [69/146 (47.3%) vs. 68/191 (35.6%), difference 11.7%, 95% confidence interval 1.1% to 22.2%; p = 0.0308]: this was similar for each component of rupture (6.8% vs. 3.7%; p = 0.1857) or repair (41.8% vs. 32.5%; p = 0.0782). USPIO enhancement was associated with reduced event-free survival for aneurysm rupture or repair (p = 0.0275). Baseline AAA diameter (p