OBJECTIVES: To Estimate the healthcare costs and clinical outcomes of various body mass index (BMI) levels over a lifetime. METHODS: A Markov model was developed to perform cost-effectiveness/utility analyses to evaluate obesity treatments including lifestyle, pharmacological and surgical interventions. User-defined cohorts of obese adults can be followed over a lifetime assessing changes in costs and life expectancy. Baseline risk factors include age, gender, race and BMI. First and second order Monte Carlo simulation techniques were utilized. To compare the outcomes of three distinct BMI groups a 20 year-old Caucasian male with varying baseline BMI (25, 35 and 45 kg.m-2) was chosen and it was assumed that no medical intervention was undertaken. The perspective was third-party reimbursement in a US setting. Mortality was adjusted for age, gender, race and BMI by Cox regression and applied to US life tables. Costs and utility values were adjusted for age, gender and BMI. US costs were derived from published resources and inflated to year 2004 figures. Reported outcomes included direct medical costs, life expectancy (LE) and quality-adjusted life expectancy (QALE) all of which were discounted at 3% according to US guidelines. RESULTS: Mean lifetime costs were $85,393 (±1545), $97,156 (±1836) and $97,542 (±2937) for BMI levels of 25, 35 and 45 kg.m-2 respectively. Undiscounted LEs were 54.43(±1.47), 51.48 (±1.37) and 42.04 (±1.69) years respectively. Lifetime costs associated with a BMI of 45 kg.m-2 from age 20 were approximately 14% higher than costs associated with BMI 25 kg.m-2. Undiscounted life expectancy was reduced by 12.39 years and discounted QALE was reduced by 3.52 years for BMI 45 kg.m-2 versus 25 kg.m-2. CONCLUSIONS: Raised BMI is associated with substantially greater direct medical costs and diminished LE and QALE. This model will allow health economic evaluations of the outcomes of various anti-obesity interventions.