Epidemiological evidence has associated dietary trans fatty acids (TFA) with heart disease. TFA are primarily from hydrogenated fats rich in elaidic acid, but dairy products also contain naturally occurring TFA such as vaccenic acid. Our purpose in this study was to compare the effects of consuming a commercially hydrogenated vegetable shortening rich in elaidic TFA (18:1t9) or a butter rich in vaccenic TFA (18:1t11) in the absence and presence of dietary cholesterol on atherosclerosis. LDL receptor deficient (LDLr(-/-)) mice were fed 1 of 8 experimental diets for 14 wk with the fat content replaced by: regular (pork/soy) fat (RG), elaidic shortening (ES), regular butter (RB), vaccenic butter (VB), or an atherogenic diet containing 2% cholesterol with RG (CH+RG), ES (CH+ES), RB (CH+RB), or VB (CH+VB). Serum cholesterol levels were elevated with cholesterol feeding (P < 0.001), whereas serum triglyceride levels were higher only in the CH+RB (P < 0.001) and CH+VB (P < 0.001) groups compared with the other 6 groups. Serum cholesterol and triglyceride levels were significantly lower in the CH+VB group than in the CH+RB group (P < 0.001). Atherosclerosis was stimulated by dietary ES compared with RG (P = 0.021), but CH+ES did not stimulate atherosclerosis beyond CH+RG alone. In contrast, VB did not induce an increase in atherosclerotic plaque formation compared with the RG and RB diets and the CH+VB diet reduced atherosclerosis compared with the other diets containing cholesterol (P < 0.01). In summary, consuming a hydrogenated elaidic acid-rich diet stimulates atherosclerosis, whereas a vaccenic acid-rich butter protects against atherosclerosis in this animal model.