To compare the prevalence of child obesity according to three criteria, a cross-sectional study was performed with 6-to-11-year-old children in eight elementary public schools in the city of São Paulo. Z scores for weight-for-height (W/H) and body mass index (BMI) were calculated from the weight and height variables. Prevalence rates for overweight in males, according to Must, Cole, and WHO criteria, were 10.92%, 10.29%, and 11.94%, respectively, and for obesity 8.17%, 13.67%, and 10.29%. For females, prevalence rates for overweight according to Must, Cole, and WHO criteria were 13.51%, 13.83%, and 13.67%, respectively, and for obesity 8.25%, 16.50%, and 11.73%. The study analyzes the variations in obesity estimates based on different criteria, highlighting the need for clinical and population studies to support the development of a single criterion. Such studies, in addition to identifying individuals with a higher probability of excess body fat, could also correlate body fat to risk of illness and death (similar to the 25 and 30 BMI cutoff points in adults).