The association between abdominal obesity and atherogenic lipid profile emerges from complex interactions of genes and environment. We aimed to explore the heritability and effects of overweight on serum lipid profile (high-density lipoprotein-cholesterol (HDL-C), HDL mean particle size, percentages of HDL(2b, 2a, 3a, 3b, and 3c,) low-density lipoprotein-cholesterol (LDL-C), LDL peak particle size and triglycerides (TGs)) in healthy, young adults. HDL-C, LDL-C, and TG were measured in 52 monozygotic (MZ) and 89 dizygotic (DZ) twin pairs, aged 23-32 years, chosen to represent a wide range of BMIs (17.6-42.9 kg/m2). Of them, 24 MZ and 26 DZ pairs were chosen at random for measurements of HDL mean and LDL peak particle sizes and percentages of HDL subspecies. The heritabilities of the lipid parameters adjusted for BMI were HDL-C 73%, HDL mean particle size 56%, HDL subspecies 46-63%, LDL-C 79%, LDL peak particle size 49%, and TG 64%. Genetic and environmental correlations between BMI and HDL-C, LDL-C, and TG were modest (0.3-0.4). Abdominal overweight (waist circumference>or=94 cm for males and >or=80 cm for females) associated with decreased HDL-C, increased LDL-C, and TG concentrations, smaller HDL mean particle size, lower HDL2b, and higher HDL3c percentages in both genders. Within MZ twins, controlling for genetic influences, within-pair differences in HDL3c percentage were associated with those in waist (r=0.46, P=0.032) and BMI (r=0.51, P=0.013). In conclusion, serum lipid parameters, including LDL peak and HDL mean particle sizes and HDL subspecies distribution are under strong genetic control. Overweight associated with significant lipid profile changes, particularly, small HDL3c increased in overweight independent of genetic influences.