Conjugated linoleic acid (CLA) mixtures demonstrated antiatherogenic properties in several animal models, including hamsters, but the mechanism of action of the main food-derived CLA isomer is unknown in this species. This study thus focused on cis-9,trans-11-CLA (rumenic acid), and its effect was compared with that of fish oil, which is known to influence several aspects of atherogenesis. Syrian hamsters were fed (for 12 wk) diets containing 20% (wt/wt) butter fat (B diet) or the same diet augmented with either 1% (wt/wt) of a cis-9,trans-11-CLA-rich oil (BR diet) or 1% (wt/wt) fish oil (BF diet). The BR diet induced the lowest aortic lipid deposition (from -30% to -45%) among the butter oil-fed hamsters. In this group, plasma also displayed a reduced non-HDL-to-HDL-cholesterol ratio (21% less than in the butter oil group) and inflammatory serum amyloid A levels (70-80%) and an improvement of anti-oxidized LDL paraoxonase activity (all P < 0.05). Compared with the B group, the beneficial effects of the BR diet could be further explained in part by preventing the high VCAM-1 expression rate, increasing (30%) ATP-binding cassette subfamily A1 expression in the aorta, and downregulating expression of inflammatory-related genes (TNF-alpha, IL-1beta, and cyclooxygenase 2, 2- to 2.8-fold, P < 0.05). This effect was partly associated with an activation of peroxisome proliferator-activating receptor (PPAR)/liver X receptor (LXR)-alpha signaling cascade. Interestingly, activation of PPAR/LXR-alpha signaling was not observed in hamsters fed the BF diet, in which the early signs of atherogenesis were increased. In conclusion, this study demonstrated that milk fat-rich cis-9,trans-11-CLA reduces the atherogenic process in hyperlipidemic hamsters.