The study purpose was to perform an obesity cost-of-illness analysis for individuals living in the province of Ontario, Canada. The participants consisted of a representative sample of 25 038 adults and 2440 adolescents (aged 12-17 years) who participated in the 2000/2001 Canadian Community Health Survey (CCHS). The CCHS data set includes measures of body mass index (BMI) (classified as normal weight, overweight or obese) and relevant covariates (age, income, smoking, alcohol, physical activity). The CCHS data set was linked to the Ontario Health Insurance Plan providers' database to obtain physician costs for 2002-2003. A two-part modelling approach was used to calculate and compare the average annual physician cost according to BMI. After adjusting for the covariates, physician costs were not significantly higher in overweight men and women compared with those with a normal weight. Physician costs were 14.7% higher in obese men and 18.2% higher in obese women than in men and women with a normal weight. Average physician costs were comparable in normal-weight and overweight/obese adolescents ($233 per year in both groups). Because Ontario operates a publicly funded healthcare system, the findings of this study have relevance for other provinces/states and countries that operate similar healthcare systems.