This paper compares the strength of intergenerational transmission of body mass index (BMI) and obesity in a sample of adoptees relative to a matched sample of biological children with similar observable characteristics. We find that BMI and obesity are strongly correlated among biological parent-child pairs, but there are no significant intergenerational associations in these health traits among adoptive parent-child pairs. The intergenerational elasticity of BMI for children to their parents is 0.2 in the matched biological sample, but indistinguishable from zero for adopted children with a standard error more than three times as large as the coefficient. Under reasonable assumptions, these findings indicate that the intergenerational transmission of BMI and obesity occurs primarily through genetic mechanisms. Additional analyses of transmission rates by parental gender and among step-parents and step-children support this conclusion. The role of determinants of BMI and obesity in the household environment in relation to our findings is discussed. Given the negative consequences of obesity on earnings and other economic measures, our results suggest that the genetic transmission of weight problems contributes substantially to intergenerational persistence in economic outcomes.