Teenagers who have overweight friends tend to develop a weight problem themselves, mounting evidence suggests.
Latest research from the US found a strong link between teenagers own weight and that of their closest peers.
A UK obesity expert said the link is likely to be causal and down to catching bad habits.
The journal Economics and Human Biology work adds weight to the notion of imitative obesity - mimicking of friends who pile on the pounds.
It looked at data on nearly 5,000 teenagers, many of whom were later followed up after two-year interval.
From this the researchers found friendships between the adolescents tended to cluster according to weight, meaning overweight children tended to hang out together.
When they looked at weight changes over time, they found having a fat friend could lead to weight gain for a child.
For example, if a 5ft 9in (1.75m) boy weighing 10st 7lb (66.6kg) - deemed a healthy weight using the Body Mass Index measurement of obesity - had a friend of the same height and weight who later gained 7lb (3.17kg), he himself would gain 2lb (0.9kg).
Although he would still be within the healthy weight range, a continued cyclical trend of gain over a period of time could tip him and his friend into obesity.
The study authors from the University of Hawaii say they cannot tell from their work whether overweight teens influence their friends to become overweight or whether obese adolescents simply choose to flock together.
If gaining weight causes one's friends to gain weight, this knowledge will be important for policy makers working on targeted campaigns, says Dr Sally Kwak and her team.
Tam Fry, of the UK's National Obesity Forum, said mounting evidence suggested the link was causative.
"Other work has shown that you take on the weight attributes of your friends more than other people surrounding you, like neighbours, even if your friends live many miles away."
He said it boiled down to shared bad habits, like eating the wrong foods and not getting enough exercise.
"If you go to dinner with your friends who are fat you are liable to eat the same foods that made them fat."
He said research showed children could also pick up bad habits from their parents.
"If your father is fat he is less likely to go out and kick a football with you and you are more likely to join him on the couch."
But he said it could work the other way, with slim children acting as good role models.
Leading by example
"The answer is, in the end, to put in a lot of education and make sure children learn at an early age about the importance of leading an active life and eating healthily.
"Maybe then children could bring good habits to friends who are obese.
"You do not stop seeing people because they are fat, but you might be able to influence them into leading a healthier lifestyle by going for a walk rather than playing computer games," he said.
A spokeswoman from Weight Concern said: "We do learn from our peers and eat with our friends, so these children may be picking up unhealthy habits.
"But I would not assume that the overweight teenagers are necessarily the ones with the bad habits. Most teenagers have unhealthy diets, but not all of them are obese.
"And most of the food consumed is still at home with the family."