Background: There is evidence suggesting that food advertising causes childhood obesity. The strength of this effect is unclear. To inform decisions on whether to restrict advertising opportunities, we estimate how much of the childhood obesity prevalence is attributable to food advertising on television (TV). Methods: We constructed a mathematical simulation model to estimate the potential effects of reducing the exposure of 6- to 12-year-old US children to TV advertising for food on the prevalence of overweight and obesity. Model input was based on body measurements from NHANES 200304, the CDC-2000 cut-offs for weight categories, and literature that relates advertising to consumption levels and consumption to body mass. In an additional analysis we use a Delphi study to obtain experts estimates of the effect of advertising on consumption. Results: Based on literature findings, the model predicts that reducing the exposure to zero would decrease the average BMI by 0.38 kg/m(2) and lower the prevalence of obesity from 17.8 to 15.2% (95% uncertainty interval 14.815.6) for boys and from 15.9% to 13.5% (13.113.8) for girls. When estimates are based on expert opinion, these values are 11.0% (7.714.0) and 9.9% (7.212.4), respectively. Conclusion: This study suggests that from one in seven up to one in three obese children in the USA might not have been obese in the absence of advertising for unhealthy food on TV. Limiting the exposure of children to marketing of energy-dense food could be part of a broader effort to make childrens diets healthier.