The amount of seriously overweight children in England has doubled in the last 10 years, according to research.
One in four kids is obese, says the Health and Social Care Information Centre after quizzing 2,000 children.
Campaigners said England was facing a "health timebomb", and the government should do more in order to stop the rise of childhood obesity by 2010.
Being badly overweight can lead to things like diabetes and heart problems in later life.
Professor Colin Waine, chairman of the National Obesity Forum, said there was a danger that kids would not live as long as their parents.
The government says it has already made big steps towards dealing with the problem through campaigns to feed kids more fruit and veg and to promote school sports.