Dietary trans fatty acids/partially-hydrogenated fat has been associated with increased risk of developing cardiovascular disease (CVD), possibly greater than predicted from changes in lipoprotein levels. To explore this issue further potential risk factors were assessed in subjects provided with each of six diets in randomized order containing as the major source of fat: soybean oil, semi-liquid margarine, soft margarine, shortening, traditional stick margarine or butter. Plasma fatty acid profiles reflected diet, with triglyceride and phospholipid subfractions affected to a greater extent than cholesteryl ester. Non-fasting LDL-cholesterol levels were 144+/-27, 141+/-27, 146+/-26, 148+/-30, 151+/-29 and 165+/-31 mg/dl (P<0.001) and total cholesterol/HDL-cholesterol ratios were 5.50+/-1.25, 5.54+/-1.50, 5.69+/-1.29, 5.82+/-1.40, 6.11+/-1.30 and 5.94+/-1.43 (P=0.011), respectively, whereas other lipoprotein levels were not significantly different. Remnant-like particles levels were unaffected by dietary fat, either in the fasting or non-fasting state. Differences in fasting insulin and glucose levels were small and would not be predicted to have a large impact on glucose homeostasis. There was no significant effect of dietary fat type on C-reactive protein levels or blood pressure. These data suggest that, as previously demonstrated, the major CVD risk factor adversely affected by dietary trans fatty acids/partially-hydrogenated fat is LDL-cholesterol levels and total cholesterol/HDL-cholesterol ratios.