Little information is available on severe obesity in childhood. This study estimates the prevalence of severe obesity in 8- to 9-year-old children resident in Italy and its association with gender, age, geographical area and parents' nutritional status and education using the World Health Organization (WHO) and International Obesity Task Force (IOTF) criteria. A nationally representative sample of grade 3 Italian students was measured in 2010 (N=42,431) using standardized instruments and methodology. Severe obesity in children was assessed using definitions provided by the WHO and by the IOTF. Prevalence was estimated within categories of sociodemographic variables and their independent effects were estimated using multivariate logistic regression. The estimated prevalence of severe obesity in 2010 was 4.5% (95% confidence interval (CI): 4.2-4.7) according to the WHO definition and 2.7% (95% CI: 2.5-2.9) with IOTF cutoffs. These values were slightly lower than those observed in 2008. The prevalence was higher in males, in 8-year-old children and in the South. Parental low education and high body mass index were strongly associated with childhood severe obesity. According to the definition used, between 30,000 and 50,000 children aged 8-9 years suffer severe obesity in Italy.