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Obesity is an important risk factor in public health. In Paris, few statistical data are available in this area. The purpose of the present study is to evaluate the prevalence of overweight and obesity in 10 years-old children attending Paris elementary schools (cours moyen deuxième année – CM2 – last level of the elementary school). 148 classes were randomly selected, gathering 3 621 schoolchildren 10 years 6 months old. 66 doctors in charge of health at school participated in the study, doing the measurements of weight, size and collecting also the weight and size at birth and at the « grande section -GS- level »(last level of the infant school, 5 years-old children) from the individual health file of the schoolchildren. The statistical analysis was based on the study of distributions of the observed Quetelet index (Q0) at the different ages, compared to French reference curves. A logistic regression analysis was performed to determine whether birth weight and GS weight predict obesity in CM2. In GS and in CM2, observed Quetelet indices are over expected values : in CM2, 22.8% of boys and 25.6 % of girls exceed the reference value Q90; the prevalence of obesity (Q0 ? Q97) is 13.4 % in boys and 13.5 % in girls. Among the variables « term », « weight at birth », weight in GS level and « gender », the weight in GS level is the only predictive factor of obesity in CM2 level. The situation in Paris appears to be serious. Preventive actions are needed at early stages to try to stop and, if possible, to reverse the present increase of overweight. In this context, school doctors have to play a prominent role.