['Vitamin B2 (riboflavin) reportedly has an antioxidant effect through superoxide dismutase (SOD) activation. However, the effect of riboflavin on abdominal aortic aneurysm (AAA) has never been investigated. In the present study, we examined the hypothesis that riboflavin has a protective effect on AAA formation in an experimental rat model.', 'The AAA model, which was induced with intraluminal elastase and extraluminal calcium chloride, was created in 36 rats. The 36 rats were divided into a riboflavin group (group R; 25\xa0mg/kg/d), and control group (carboxymethyl cellulose). Riboflavin administration by gastric gavage once per day was started at 3\xa0days before aneurysm preparation. On day 3, SOD activity in aneurysm walls was assayed. On day 7, reactive oxygen species (ROS) levels were semiquantified by dihydroethidium staining, and the oxidation product of DNA produced by ROS, 8-hydroxydeoxyguanosine (8-OHdG), was measured by immunohistochemical staining. Histopathologic examination (hematoxylin/eosin and elastica Van Gieson staining) was performed on day 28, and the AAA dilatation ratio was calculated to evaluate the protective effect of riboflavin.', 'On day 3, SOD activity was significantly increased in aneurysm walls by riboflavin administration (370\xa0± 204 U/mL in normal, 334\xa0± 86 U/mL in control, 546\xa0± 143 U/mL in group R; P\xa0= .021). On day 7, ROS levels and 8-OHdG-positive cells in aneurysm walls were significantly decreased by riboflavin treatment (ROS levels: 1.0\xa0± 0.1 in normal, 4.5\xa0± 0.4 in control, 3.1\xa0± 0.5 in group R, P\xa0< .01; 8-OHdG-positive cells: 30\xa0± 2 cells in normal, 148\xa0± 20 cells in control, 109\xa0± 15 cells in group R, P\xa0< .01). Riboflavin treatment significantly reduced matrix metalloproteinase (MMP)-9 messenger RNA expression in aneurysm walls (relative expression: MMP-9: 0.4\xa0± 0.7 in normal, 2.6\xa0± 1.3 in control, 0.5\xa0± 0.3 in group R, P\xa0< .01). On day 28, the aortic walls were less dilated and had higher elastin content in group R than in control (dilatation ratio: 194.9%\xa0± 10.9% in control, 158.6%\xa0± 2.5% in group R; P\xa0<.01).'] Does endogenous superoxide dismutase activation by oral administration of riboflavin reduce abdominal aortic aneurysm formation in rats?