The number of obese children in Scotland is running at double the UK average, according to NHS statistics.
The number of obese children is increasing across all ages
More than a third of 12-year-olds were overweight in the 2004/05 school year, while 19.4% were obese and 11.2% were classed as severely obese.
Some 20% of those aged three-and-a-half were overweight, 8.6% were obese and 4% were severely obese.
The Information Statistics Division of NHS Scotland said many of the figures were double the expected UK average.
"At all ages, the proportion of Scottish children who were estimated to be overweight, obese and severely obese were higher than expected," said a spokesman.
"Being overweight or obese during childhood can lead to physical and mental health problems in later life, such as heart disease, diabetes, osteoarthritis, back pain, low self-esteem and depression."
He said levels of obesity among schoolchildren had risen over the last five years.
The greatest increase was in primary seven, where the proportion of overweight children rose by almost 4% to 34.1%.
There were increases of about 3% in the numbers of obese and severely obese 12-year-olds.
Deputy Health Minister Lewis Macdonald said it was worrying that obesity levels were higher for children living in deprived areas.
"Tackling health inequalities is a top priority for us," he said.
The executive was working with a wide range of organisations, including manufacturers, schools and the health service, to promote healthy choices, he added.
"Changing to a healthier lifestyle will not happen for many families overnight, but we are committed to tackling this issue in the long-term.
"Scotland is not alone in experiencing a rapid rise in obesity in the last two decades, all developed countries are experiencing a similar trend, but we do want to be in the forefront in tackling it."
Tory health spokeswoman Nanette Milne said the figures for children aged three-and-a-half were the most troubling.
She said: "This sends a clear message that we need to start educating people about the dangers of an unhealthy lifestyle before they are born, starting with mothers in ante-natal classes.
"There needs to be an awareness of the damaging effects created by a poor diet and insufficient exercise.
"It is a parental duty to provide balanced, nutritional meals and to encourage an active lifestyle."
Scottish National Party health spokeswoman Shona Robison said: "These shocking figures clearly show Scotland is still lagging well behind the UK in tackling the problem of child obesity.
"Obviously, the executive has failed to end the junk food, couch potato culture that is all to prevalent in Scotland."