INTRODUCTION: Previous studies place Portugal among the five countries with the highest prevalence of childhood obesity in Europe. This paper describes the prevalence of thinness, overweight and obesity in Portuguese children of 6-8 years of age, based on the first data collection from Childhood Obesity Surveillance Initiative Portugal, which took place during the 2007/2008 school year. METHODS: This study uses a semi-longitudinal design with repeated cross-sectional national representative samples. Specific prevalence of overweight (including obesity) and obesity was determined using three different diagnostic criteria. Across the seven geographic regions, 3765 children were enrolled from 181 schools; 50.3% of participants were males. RESULTS: Using the International Obesity Task Force reference, the prevalence of thinness, overweight and obesity were 4.8%, 28.1% and 8.9%, respectively; using the Center for Disease Control and Prevention reference they were 2.1%, 32.2% and 14.6%, respectively; and according to the World Health Organization reference, they were 1.0%, 37.9% and 15.3%, respectively. Univariate analysis showed a higher risk of obesity in older children, in boys and in the Azores region. The islands of Madeira and the Azores were the regions with the highest prevalence of overweight at 39.4% and 46.6%, respectively, and Algarve was the one with the lowest (21.4%). CONCLUSION: These findings demonstrate the need for urgent action in Portugal and provide policy-makers with comprehensive and detailed information to assist with this.