Obesity is associated with adverse changes in plasma lipoprotein metabolism, but it is not known completely how this association is modified by genetic factors. We assessed the contribution of obesity to serum lipid and lipoprotein levels and lipid metabolizing enzyme activities by examining 23 identical twin pairs (9 male, 14 female) who had, on the average, an 18-kg intrapair difference in BW. Compared with lean co-twins, obese co-twins had approximately 20% higher low-density lipoprotein (LDL) cholesterol (P < 0.01), 20% lower high-density lipoprotein2 cholesterol (P = 0.010), and 90% (men) or 35% (women) higher (P < or = 0.06) total, very-low-density lipoprotein and LDL triglycerides. The pairs were divided into subgroups by the gender-specific median value of abdominal visceral fat (AVF) area in the obese co-twin and by apolipoprotein E 4 phenotype. The intrapair differences in serum cholesterol fractions were similar in twin pairs with high or low AVF, whereas only high AVF pairs showed significant differences in triglyceride fractions. The greatest intrapair differences in total, very-low-density lipoprotein and LDL triglycerides were observed in apolipoprotein E 4-positive pairs expressing high AVF. Compared with lean co-twins, lecithin cholesterol acyltransferase activity was 18% higher (P < 0.001) and hepatic lipase activity was 38% higher (P = 0.016) in obese co-twins with high AVF. When genetic factors are identical, obesity is associated with an atherogenic lipid profile, especially in subjects with high visceral fat accumulation.