OBJECTIVES: There is a substantial body of epidemiological evidence relating body-mass index (BMI) to increased risk of mortality in subjects with type-2 diabetes mellitus (T2DM). Cardiovascular (CV) and mortality risk equations typically incorporate the effects of elevated BMI via the inter-relationship between modifiable CV risk factors (cholesterol and systolic blood pressure) and BMI; this approach may underestimate true mortality risk.  Therefore, the objective of this study was to assess by how much existing risk equations underestimate the risk of mortality as a function of increasing levels of BMI. METHODS: We projected life expectancy (LE) using the IMS CORE Diabetes Model (CDM), a validated and widely used simulation model designed to predict the health care costs and benefits associated with diabetes.  Projected LE was obtained using patient level data (PLD) for subjects with T2DM from NHANES.  CV and mortality risk was assessed using UKPDS risk equations and additional BMI cause specific mortality included via results from published prospective analyses of 900,000 adults.  Multiple linear regression analysis of simulation output relating LE to levels of BMI was conducted using R. RESULTS: Results were obtained for 1855 subjects with mean age 63.6 years, 53% male; 16% current smokers; duration of diabetes 9.5 years; HbA1c 7.4%; SBP 135mmHg; total cholesterol 195mg/dl and BMI 30.6kg/m2. Using UKPDS mortality risk the predicted mean discounted LE was 11.8 years with each unit increase in BMI associated with a 0.06 years less life expectancy. Using BMI cause specific mortality decreased mean discounted LE to 10.85 years; with each unit increase in BMI linearly associated with a 0.2 years reduction in LE. CONCLUSIONS: Failure to adequately incorporate the deleterious effect of increasing levels of BMI on risk of mortality may substantially distort the health economic assessment of type-2 diabetes specific therapies; particularly those associated with the avoidance of weight gain or weight loss.