Heritability estimates of human body fatness vary widely and the contribution of body composition methodol-ogy to this variability is unknown. The effect of body composition methodology on estimations of genetic and environ-mental contributions to body fatness variation was examined in 78 adult male and female monozygotic twin pairs reared apart or together. Body composition was assessed by six methods – body mass index (BMI), dual energy x-ray absorpti-ometry (DXA), underwater weighing (UWW), total body water (TBW), bioelectric impedance (BIA), and skinfold thick-ness. Body fatness was expressed as percent body fat, fat mass, and fat mass/height² to assess the effect of body fatness expression on heritability estimates. Model-fitting multivariate analyses were used to assess the genetic and environmental components of variance. Mean BMI was 24.5 kg/m² (range of 17.8-43.4 kg/m²). There was a significant effect of body composition methodology (p<0.001) on heritability estimates, with UWW giving the highest estimate (69%) and BIA giv-ing the lowest estimate (47%) for fat mass/height². Expression of body fatness as percent body fat resulted in significantly higher heritability estimates (on average 10.3% higher) compared to expression as fat mass/height² (p=0.015). DXA and TBW methods expressing body fatness as fat mass/height² gave the least biased heritability assessments, based on the small contribution of specific genetic factors to their genetic variance. A model combining DXA and TBW methods re-sulted in a relatively low FM/ht² heritability estimate of 60%, and significant contributions of common and unique envi-ronmental factors (22% and 18%, respectively). The body fatness heritability estimate of 60% indicates a smaller contri-bution of genetic variance to total variance than many previous studies using less powerful research designs have indi-cated. The results also highlight the importance of environmental factors and possibly genotype by environmental interac-tions in the etiology of weight gain and the obesity epidemic.