OBJECTIVES: To present an overview of the burden of obesity in adults by using the richness of a Canadian population health survey linked to Ontario administrative databases. METHODS: The records of all Ontarians who participated in the Canadian Community Health Survey (CCHS), cycle 1.1 (2000/2001) and provided consent to data linkage with administrative databases were linked to the Ontario Health Insurance Program (OHIP) claims database and the Discharge Abstract Database (DAD) In-Patient and Day Procedure database. Prevalence of obesity in this adult population was documented using the body mass index (BMI) calculated by Statistics Canada for adults aged 20-64 years of age. Socio-demographics (e.g. age, gender, education), medical characteristics (e.g. comorbidities), health related quality of life (Health Utility Index 3), self-reported health and one-year physician and hospitalization costs were described per BMI category (i.e. underweight, normal weight, overweight and obese). RESULTS: More than 50% of adult participants were either overweight or obese in 2000/2001. When classified by BMI categories, obese individuals were more likely to be older, males, have more comorbidities, a lower quality of life and higher one-year medical costs. A Generalized Linear Model (GLM) indicated that age, gender, obese, being inactive and smoker increased significantly the total costs. Compared to normal weight adults, being underweight or obese were also positive predictors of costs. Health-related quality of life, income or being overweight had no impact on costs. CONCLUSIONS: These results suggest that the burden of obesity in Ontario is considerable.