To estimate the prevalence of childhood overweight and obesity among Italian schoolchildren and to examine geographic differences and present and future implications for health care, we used data from a nationwide representative survey performed in May 2008 among third-grade students in 18 of Italy's 21 regions. Cluster sampling was used to identify classes for participation. The study population included all children aged 8–9 years whose parents agreed to opt-out consent. Parents, children and teachers completed brief questionnaires, and children were weighed and measured by trained staff using standardized equipment. Consent was obtained for 97% of 50 197 third-graders, of whom 44 676 (89%) met study inclusion criteria. Obesity levels (defined using International Obesity Task Force cut-offs) ranged from 7.5% (95% confidence intervals 6.7–8.2) in the north to 16.6% (95% confidence intervals 15.8–17.4) in the south. Behaviours known to be associated with obesity also showed geographic differences. The estimated burden of obesity-related pathologies also increased from north to south. These findings suggest the need for community as well as individual interventions in all areas of the country but with particular attention to the south.