The consumption of high-fat hamburger enriched with SFA and trans-fatty acids may increase risk factors for coronary vascular disease, whereas hamburger enriched with MUFA may have the opposite effect. Ten mildly hypercholesterolaemic men consumed five, 114 g hamburger patties per week for two consecutive phases. Participants consumed high-SFA hamburger (MUFA:SFA = 0.95; produced from pasture-fed cattle) for 5 weeks, consumed their habitual diets for 3 weeks and then consumed high-MUFA hamburger (MUFA:SFA = 1.31; produced from grain-fed cattle) for 5 weeks. These MUFA:SFA ratios were typical of ranges observed for retail ground beef. Relative to habitual levels and levels during the high-MUFA phase, the high-SFA hamburger: increased plasma palmitic acid, palmitoleic acid and TAG (P < 0.01); decreased HDL cholesterol (HDL-C) and LDL particle diameter percentile distributions (P < 0.05); and had no effect on LDL cholesterol or plasma glucose (P>0.10). Plasma palmitoleic acid was positively correlated with TAG (r 0.90), VLDL cholesterol (r 0.73) and the LDL:HDL ratio (r 0.45), and was negatively correlated with plasma HDL-C (r - 0.58), whereas plasma palmitic, stearic and oleic acids were negatively correlated with LDL particle diameter (all P <or= 0.05). Because plasma palmitoleic acid was derived from Delta9 desaturation of palmitic acid in liver, we conclude that alterations in hepatic stearoyl-CoA desaturase activity may have been responsible the variation in HDL-C and TAG caused by the high-SFA and high-MUFA hamburgers.