Childhood overweight and obesity have become a global public health problem. This study aims to determine the prevalence of these health conditions in children studying in government schools in the two cities of Buraidah and Unaizah of the Qassim Region, Saudi Arabia. The key question that we examined was whether the children enter the school as overweight or become overweight after entering the school. Using a cross-sectional, observational study design, a random sample of 874 school children between 6– to 10 years was enrolled in 2010/2011. A structured questionnaire was used for collecting data. Weight and height were measured, and the body mass index (BMI) was categorized. Based on the World Health Organization (WHO) definition, the overall prevalence of overweight and obesity was 12.8% and 10.1%, respectively. Girls had a higher prevalence of overweight (18.4%) and obesity (15.6%) than boys. Overweight tendency increased dramatically from 7.6% in Grade 1 to 19% in Grade 4. Similarly, obesity also increased progressively after entering the school. Our study suggests that overweight and obesity are mostly acquired after entering the school. Public health program are, therefore, required to promote a healthy lifestyle from the early years of schooling.