OBJECTIVE The study aimed to assess the prevalence of obesity and the related behavioral risk factors among children in Beijing. METHODS Data were collected as part of a 2004 comprehensive study on obesity and metabolic syndrome among 21 198 children, aged 2-18 years old in Beijing. Four local and international references for the classification of childhood overweight and obesity were compared. RESULTS The combined prevalence of obesity and overweight among children, aged 2-18 years old, was 18.7%, 21.4%, and 20.1% based on the International Obesity Task Force (IOTF) reference, new World Health Organization (WHO) growth standard, and the US 2000 Center for Disease Control and Prevention Growth Charts, respectively; and it was 21.7%, according to the local Chinese body mass index (BMI) reference for school-age children (aged 7-18 years). Physical inactivity, less sleep duration, higher consumption of alcohol, snack food and Western fast food and more screen time were associated with overweight and obesity (p<0.05). CONCLUSIONS Over 20% of children and adolescents in Beijing are overweight or obese. Effort should be made to combat the growing obesity epidemic.