Introduction Obesity childhood has been identified as a major public health problem and is associated with increased risks of adult obesity and chronic diseases. In 2007 the Italian Ministry of health promoted a national school-based nutritional surveillance system, based on biannual cross-sectional surveys and coordinated by the National Institute of Health. The aim was to estimate the prevalence of overweight and obesity among Italian schoolchildren and to examine the main risks behaviour. Methods Cluster sampling in all Italian regions was used to identify the 3rd primary classes for participation. Parents, children and teachers completed brief questionnaires to study the risks behaviour of children. The body mass index was calculated and cut-offs of the International Obesity Task Force were used to define overweight and obesity. Results In 2010, 42549 students were weighed and measured by trained local health staff. 34.2% of the children were overweight or obese. Data on dietary habits shows that 9% of the children did not have breakfast and 68% consumed mid-morning calorific snacks. Furthermore 23% of parents said their children did not consume fruits and vegetables daily. 22% of the children practiced usually physical activity for one hour a week and only 25% did it for 4 hours or more. Conclusions This surveillance system collected information on the behaviour of children and their weight status, and it is a valuable tool to monitor the evolution of the obesity epidemic.