There has been a four-fold increase in the number of children and teenagers admitted to hospital for obesity-related conditions in the last decade, doctors in England and Wales warn.
In 2009, nearly 4,000 young people needed hospital treatment for problems complicated by being overweight compared with just 872 in 2000.
Rates of obesity surgery also went up, especially for teenage girls.
Doctors say the UK has the highest rate of child obesity in Western Europe.
Obesity has been linked with serious illnesses during childhood and an increased risk of developing conditions, such as type-2 diabetes, asthma and breathing difficulties during sleep.
National surveys in England suggest about three in 10 two-to-15-year-olds are overweight, while 14-20% are obese.
A team led by Dr Sonia Saxena, of Imperial College London, analysed statistics on all NHS admissions for obesity - as a primary cause or alongside conditions that had been complicated by obesity - in hospitals in England and Wales over a 10-year period in patients aged five to 19.
Admissions were more common in girls than boys, the team reported in the journal Public Library of Science ONE.
Meanwhile, the number of cases of surgery for obesity rose from just one in 2000 to 31 in 2009, with the majority in teenage girls.
Over the whole 10-year period, a total of 20,885 young people were treated in hospital for obesity-related conditions.
Nearly three-quarters of cases involved problems complicated by being overweight, such as asthma, sleep apnoea, and pregnancy complications.
The researchers, from Imperial College and the Medical University of South Carolina, say that while part of the increase is probably due to better monitoring of obesity in children, the condition is imposing greater challenges for hospitals.
Dr Saxena said the UK was now seeing serious consequences of people being obese in their teenage years and early adulthood.
"We are seeing - through obesity - an increasing number of children with conditions that we previously diagnosed in adulthood... [and which] are now being diagnosed in childhood," she told BBC News.
"What's new about our paper is that we're actually showing it's not a ticking time-bomb - the time-bomb is exploding within the early life course, so in other words in the teenage years. That's where it's becoming manifest."
The UK has the highest rate of child obesity in Western Europe, which is estimated to cost the NHS about £4.2bn a year.
The Royal College of Paediatrics and Child Health said there was a need for urgent action, but there was no "silver bullet" solution.
Officer for health promotion, Prof Mitch Blair, said: "We need to look seriously at how fast food is marketed at children and consider banning junk food prior to the 21:00 watershed, limiting the number of fast food outlets near schools, and making sure children are taught the importance of a healthy, balanced diet and how to cook nutritious meals from an early age at school."
Amy Thompson, senior cardiac nurse at the British Heart Foundation, said: "These are very worrying findings that shed more light on a growing threat to the heart health of this nation.
"We know obese children are more likely to become obese adults who are then at greater risk of having a heart attack or stroke. We must encourage the next generation to make healthier lifestyle choices and help them eat a balanced diet and stay active.
"Ensuring children and teenagers are a healthy weight today means healthier hearts tomorrow."