Some children who are very thin are being misdiagnosed as anorexic when they have the gut disorder Crohn's disease, a leading expert has warned.
Crohn's disease affects 100,000 people in Britain
Child health specialist Professor Ian Booth told a conference that treatment can be delayed for months as a result.
He said teenagers with Crohn's - an inflammation of the digestive tract - could present with growth failure but no digestive symptoms.
Professor Booth said doctors should be aware Crohn's was a possible diagnosis.
The diagnosis problems arise when children and teenagers are extremely thin and failing to thrive, he told a British Society of Gastroenterology meeting in Birmingham.
Doctors may assume the patients have anorexia when they are actually having problems eating and digesting food because of Crohn's disease.
Crohn's usually affects the small intestine. People with the condition may develop obstructions in their bowel, making digesting food painful.
Professor Booth told the BBC News website: "This is an issue which is numerically very small, but individually very important.
"Growth failure in the absence of intestinal symptoms can be an important presentation of Crohn's in adolescents.
"The other important presentation is in wasting - as in malnutrition - so much so that presenting this way in adolescence, it is sometimes confused with anorexia nervosa."
He said concerns about misdiagnosing children had first been raised 40 years ago, but cases were still being seen.
Professor Booth cited the case of a girl who had suffered from low grade intestinal symptoms for several years.
Her main symptom was a refusal to eat. She also had growth failure.
"She was diagnosed by psychiatrists as anorexic and was admitted for in-patient treatment," he said.
"She was exposed to a fairly punitive style management of anorexia. It was eventually recognised after about six months in hospital that she had small bowel Crohn's disease."
He added: "Crohn's disease can present as growth failure or sometimes masquerade as anorexia nervosa.
"But mistakenly treating them for anorexia can cause damage to their psyche."
Richard Driscoll, director of the National Association for Colitis and Crohn's Disease, said his organisation received several reports each year of patients being misdiagnosed as anorexic.
"Young teenagers may be losing weight and then they stop eating because their condition makes it painful to eat.
"If they haven't got any other symptoms, their condition can be classed as anorexia.
"It's not something that happens frequently, but it does occur."