Background. The US has the highest prevalence of obesity and one of the lowest life expectancies among OECD countries. While it is plausible to assume that these two phenomena are related, no previous attempt has been made to identify the connection between them. Our paper uses primary data on body mass index (BMI) in 16 countries and detailed information on the mortality risks of obesity to estimate the effect of international differences in obesity on comparative levels of longevity. Methods. We estimate the fraction of deaths from all causes attributable to obesity by country, age and sex. We then re-estimate life tables in 2006 by removing deaths attributable to obesity. To allow for the possibility of a secular decline in obesity risks, we employ two alternative sets of risks drawn from a more recent period than the baseline risks. Results. In our baseline analysis, we estimate that US life expectancy at age 50 in 2006 was reduced by 1.54 years (95% condence interval (CI) 1.37-1.93) for women and by 1.85 years (1.62-2.10) for men as a result of obesity. Relative to higher life expectancy countries, allowance for obesity reduces the US shortfall in life expectancy by 42% (36-48) for women and 67% (57-76) for men. Using obesity risks that were recorded more recently, differences in obesity still account for a fifth to a third of the shortfall of life expectancy in the US relative to longer-lived countries. Conclusions. The high prevalence of obesity in the US contributes substantially to its poor international ranking in longevity.