OBJECTIVES: To determine the average body mass index (BMI) and the prevalence of overweight and obesity among people aged 20 to 64 years, to identify sociodemographic, lifestyle and health variables that correlate with overweight and obesity, and, through a comparison of the results with those from an earlier survey, to determine whether prevalence has changed over time. DESIGN: Cross-sectional survey. SETTING: Ontario. PARTICIPANTS: The 1990 Ontario Health Survey surveyed 61,239 people representative of the Ontario population. The authors' analyses were restricted to those aged 20 to 64 years, excluding pregnant women. In the multivariate analyses they included only people with no missing values for any of the variables in the models (n = 26,306). OUTCOME MEASURES: BMI (weight in kilograms divided by height in metres squared) was used to measure healthy weight (BMI between 20 and 25), overweight (BMI greater than 25) and obesity (BMI greater than 27). RESULTS: The prevalence of obesity among men and women was 33.6% and 22.8% respectively (adjusted odds ratio [OR] 1.78, 95% confidence interval [CI] 1.63 to 1.95). There was a positive relation with age (adjusted OR 1.53 [95% CI 1.24 to 1.89] for age 25 to 29 years and 2.78 [95% CI 2.20 to 3.51] for age 50 to 54 years compared with age 20 to 24 years) and an inverse relation with education level (postsecondary education v. primary education: adjusted OR 0.65 [95% CI 0.54 to 0.79]). Analysis of birthplace showed that the prevalence of obesity was lowest among those born in Asia (compared with Canadian born: adjusted OR 0.36 [95% CI 0.27 to 0.47]). The prevalence was higher among former smokers than among those who had never smoked (adjusted OR 1.20 [95% CI 1.18 to 1.22]). People with more health problems and those who rated their health as fair or poor were more likely to be obese. The estimates of the prevalence of obesity were higher than those reported in the 1985 Health Promotion Survey for both sexes in all three age groups examined. CONCLUSIONS: These self-reported data indicate that overweight and obesity remain important health problems in Ontario, and the prevalence appears to be increasing.