The composition of intraluminal thrombus (ILT) is uniquely identified by MRI and has been suggested as a marker of abdominal aortic aneurysm (AAA) growth. However, the natural history of ILT progression is still unknown. This study followed 25 AAA patients over 19±9 months using repeated high resolution black-blood MRI. We found baseline ILT types did not predict AAA growth, however, AAAs with new ILT formation or fresher ILT during follow-up grew 3 times faster than AAAs without ILT change or older ILT (4.0±2.3mm/year vs. 1.3±2.5 mm/year, p=0.009). Monitoring ILT change provides new insights into the AAA risk assessment.