Childhood obesity rose sharply in the 1990s. The routine school-based medical and developmental screening of 5- to 6-year-olds provides an opportunity to study its prevalence. This cross-sectional study involved a representative sample of 1,008 children in Haute-Savoie in 2008. The prevalence of obesity (grades 1 and 2 of international standards) is 9.7%. It is 15.2% among students in disadvantaged areas compared to 9.1% among other children, representing a relative risk of 1.7. In contrast, prevalence does not differ by gender. Severe underweight (grades 2 and 3) affects only 1.0% of children. In multivariate analysis, only three factors are associated with obesity: having two parents originally from countries other than metropolitan France and Switzerland, bringing a snack for the afternoon recess, and having a family history of overweight. Socioeconomic characteristics, lifestyle habits, diet, and well-being at school do not appear to be linked to obesity in this study. A comparison with an initial retrospective departmental study conducted in 2003 among sixth-grade students shows that the prevalence of obesity among 5- to 6-year-olds in Haute-Savoie stabilized between 1998 and 2008, which is consistent with recent national studies. (R.A.)