['The purpose of these experiments was to test the hypothesis that dietary phytoestrogens would diminish experimental aortic aneurysm formation.', 'Six-wk-old C57BL/6 mice were divided into groups, fed either a diet with minimal phytoestrogen content or a regular commercial rodent diet with high phytoestrogen content for 2\xa0wk. At the age of 8\xa0wk, aortic aneurysms were induced by infusing the isolated infrarenal abdominal aorta with 0.4% elastase for 5\xa0min. Mice were recovered and the diameter of the infused aorta was measured at postoperative days 3, 7, and 14. Abdominal aorta samples were collected for histology, cytokine array, and gelatin zymography after aortic diameter measurement. Blood samples were also collected to determine serum phytoestrogens and estradiol levels. Multiple-group comparisons were done using an analysis of variance with post hoc Tukey tests.', 'Compared with mice on a minimal phytoestrogen diet, mice on a regular rodent diet had higher levels of serum phytoestrogens (male, 1138\xa0±\xa0846\xa0ng/dL; female, 310\xa0±\xa0295\xa0ng/dL). These serum phytoestrogen levels were also much higher than their own endogenous estradiol levels (109-fold higher for males and 35.5-fold higher for females). Although aortic diameters of female mice were unaffected by the phytoestrogen concentration in the diets, male mice on the regular rodent diet (M+ group) developed smaller aortic aneurysms than male mice on the minimal phytoestrogen diet (M- group) on postoperative day 14 (M+ 54.8\xa0±\xa08.8% versus M- 109.3\xa0±\xa037.6%; P\xa0<\xa00.001). During aneurysm development (postoperative days 3 and 7), there were fewer neutrophils, macrophages, and lymphocytes in the aorta from the M+ group than from the M- group. Concentrations of multiple proinflammatory cytokines (matrix metalloproteinases [MMPs]; interleukin 1β [IL-1β]; IL-6; IL-17; IL-23; monocyte chemoattractant protein-1; regulated on activation, normal T cell expressed and secreted; interferon γ; and tumor necrosis factor α) from aortas of the M+ group were also lower than those from the aortas of the M- group. Zymography also demonstrated that the M+ group had lower levels of aortic MMP-9s than the M- group on postoperative day 14 (P\xa0<\xa00.001 for pro-MMP-9, P\xa0<\xa00.001 for active MMP-9).'] Do dietary phytoestrogens inhibit experimental aneurysm formation in male mice?