A woman has dramatically gained weight after a stool transplant from her daughter, doctors report.
It is a genuine medical procedure to transplant healthy bacteria into a diseased gut, but US doctors think it may have affected her waistline.
She quickly gained 36lb (16kg) and is now classed as obese, the case report in Open Forum Infectious Diseases says.
A UK expert said the link between gut bugs and obesity was still unclear.
A faecal microbiota transplant - also referred to by some as a "transpoosion" - is like an extreme version of a probiotic yogurt.
The aim is to introduce good bacteria into the gut and it was officially backed by the UK health service last year.
It is used when people have stubborn Clostridium difficile infection in their bowels.
This can cause vomiting, diarrhoea and abdominal pain and cannot always be treated with antibiotics.
The 32-year old woman, who has not been indentified, had an infection that could not be treated with even the most powerful antibiotics.
Dr Colleen Kelly, from the Medical School at Brown University, said the option of a faecal transplant was discussed and the woman wanted to use a relative - her daughter.
The daughter was overweight at the time and was on her way to becoming obese.
The procedure did clear the woman's infection.
But Dr Kelly told the BBC News website: "She came back about a year later and complained of tremendous weight gain.
"She felt like a switch flipped in her body - to this day she continues to have problems."
She started with a Body Mass Index of 26. Sixteen months after the procedure she had a BMI of 33 and three years after it, a BMI of 34.5.
Previous research has shown that transplanting gut bacteria from obese people into mice led to the animals gaining weight.
Dr Kelly said limited conclusions could be drawn from a single patient, but called the case a warning as "there's not a lot on safety evidence out there".
Dr Kelly has now changed her practices and "as a result I'm very careful with all our donors don't use obese people".
Dr Andreas Karatzas, from Reading University, said: "You have to bear in mind that this person was saved.
"If you run the risk of losing a patient, you don't bother about what could happen 20 years later."
However, he said the evidence that gut bacteria affected human waistlines was still inconclusive.
"There is some evidence in animals, but we have to be careful - it is a different organism. Just because it happens in animals doesn't mean it happens in humans as well."