A recent review of the literature concluded that advertising of foods on television may influence children's food choices and encourage unhealthy diets, but the review acknowledged there was a lack of clear evidence in coming to this conclusion. The present paper examines ecological evidence for a link between advertising to children and the risk of overweight using data from surveys of advertising on children's television and estimates of the prevalence of overweight among children, in the USA, Australia and eight European countries. A significant association was found between the proportion of children overweight and the numbers of advertisements per hour on children's television, especially those advertisements that encourage the consumption of energy-dense, micronutrient-poor foods (r = 0.81, P < 0.005). A weaker, negative association was found between the proportion of children overweight and the number of advertisements encouraging healthier diets (r = −0.56, P < 0.10). The quantity of advertising on children's television appears to be related to the prevalence of excess body weight among children. Furthermore, the content of the advertising appears to have a specific effect. The findings justify the need for taking precautionary measures to reduce children's exposure to obesogenic marketing practices.