OBJECTIVES: Obesity, defined as body mass index (BMI)>30, is a major contributor to increased morbidity, mortality, and healthcare expenditures.   There is a tendency for obese individuals to continually gain weight over their lifetimes.  However, neither the strength of this tendency nor its economic consequences are well understood.  We constructed a health outcomes model from a societal perspective to assess this. METHODS: Our model projected lifetime trajectories for BMI, death, cost, and quality-adjusted life years (QALYs). Using these projections, we computed the net economic value of potential weight loss for an individual given baseline characteristics:  Age, Sex, and BMI.  We used existing literature and public datasets to estimate the parameters of the model. For the BMI trajectory, we utilized existing literature on pooled databases of the individuals’ BMI over time.   For the probability of death, we constructed life tables by estimating the 5-year probability of death using a logistic regression model using data from the National Health Institute Survey linked with National Death Index (NHIS-NDI) between 1997 and 2005.  For costs and QALYs, we use Medical Expenditure Panel Survey Panels 6-10 to estimate hedonic linear regressions in each period.  Costs and QALYs were discounted at 3%. RESULTS: In the base case— for a 45 year-old female—the expected age of death for baseline BMI of 25, 35, or 45 was 83, 80, and 77, respectively.  The projected difference in discounted lifetime healthcare costs between this non-obese (BMI = 25) person and someone BMI = 45 is about $26,000. If the loss in QALYs were valued at $100,000/QALY, the net economic value loss is projected at $271,000.   CONCLUSIONS: Obesity is associated with higher medical costs, lower quality of life, and reduced life expectancy.  The societal cost of delivering effective weight loss interventions to obese Americans should be considered in the context of these lifetime outcomes.