Methods for detecting abdominal aortic aneurysm (AAA) or predisposition to AAA in a apoE subject, methods for monitoring the efficacy of treatment of AAA in a subject, and methods for evaluating the severity of AAA or risk of AAA in a subject involve measuring the amount of tetrahydrobiopterin (H4B) present in the test sample and comparing it to the amount of H4B present in a standard or previous test sample. A decreased amount of H4B present in the test sample compared to the standard is indicative of AAA or predisposition to AAA. Treatment can be administered to the subject prior to a second time point, and an increased amount of H4B present in the second test sample compared to the first test sample is indicative of effective treatment of AAA. Candidates can be identified for further testing or monitoring for AAA, and/or for treatment for AAA.