Many more teenagers in Britain would be considered obese if their waist size was taken into consideration by doctors, claims a study.
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Researchers from the Institute of Child Health in London say that "body mass index", the traditional way to assess children, may be misleading.
As a result, the prevalence of obesity in the young may have been "systematically underestimated", say the researchers.
The body mass index of children is already known to be increasing at an alarming rate, fuelling fears of an "epidemic" of obesity.
However, the waist circumference of children is also increasing - and at a much faster rate.
Some doctors believe that this may be a more accurate predictor of future ill health than body mass index.
They think that fat accumulating around the waistline is a far better guide to the total amount of fat on the body.
Even if a person has a high mass in relation to their height, there is no guarantee tha much of this weight is not composed of muscle.
A large waist circumference in adulthood is associated with a wide range of illnesses, including heart disease and type 2 diabetes.
The speed of increase was greater in 11 to 16-year-old girls than in boys of the same age.
A survey carried out in 1977 for the British Standards Institute suggested that the average waistline of the 16-year-old girl in the UK was 66cm, and 72cm for boys.
In the National Diet and Nutrition Survey of 1997, the average was 73cm for girls and 80cm for boys.
Dr David McCarthy, senior lecturer in human nutrition at London Metropolitan University, and one of the report's authors, said: "There are drawbacks to using body mass index - it doesn't tell you how the fat is distributed.
"These children may be storing up problems for their health later in life."
He said that surveys suggested that the children of today are actually eating less than their counterparts in the 1970s - which meant that their lowered activity levels were to blame for the increase in obesity among children.
Dr Elizabeth Poskitt, a nutrition researcher from the London School of Hygiene and Tropical Medicine, told the BBC that there was no "perfect answer" to guaging the obesity of children.
She said: "Nobody has shown how much of waist circumference is actually fat.
"A lot depends on the body shape of the child - and how tall they are, as taller children tend to be broader around the waist.
"The really important thing is to develop effective programmes to prevent and manage obesity - and perhaps as a society develop a less negative attitude to those who are obese so they can admit the problem without shame."