A new complication has been seen in a patient with kidney disease who received stem cell therapy, scientists have warned.
Stem cells were injected into the kidney, but the patient suffered tissue damage and died from an infection.
The Canadian and Thai researchers said the findings published in the Journal of the American Society of Nephrology showed caution was needed.
Experts said there was a gap between research and treatment.
Many scientists hope stem cell therapy can be used to treat a wide range of diseases.
It has been shown that it is possible to reprogram adult stem cells, taken from bone marrow, to become a range of specific cell types - including kidney cells.
And animal studies have indicated that injecting stem cells directly into organs, including the kidney, is safe.
The patient in this case had been treated by a private clinic.
Researchers from Chulalongkorn University in Bangkok, Thailand, and Paul Scott Thorner, from the University of Toronto, were involved in removing and analysing the kidney.
They found that the patient had not benefited at all from the treatment, but had actually developed tissue damage called angiomyeloproliferative lesions at the injection sites. These were found to be clusters of blood vessels and bone marrow cells.
Dr Duangpen Thirabanjasak, from Chulalongkorn University, who led the research, said: "This type of lesion has never been described before in patients, and we believe that this is either formed directly by the stem cells that were injected or that the stem cells caused these masses to form."
And she warned that, because they had not been seen before, no-one knew how the lesions might have developed over time.
The authors conclude that their findings should serve as a warning to clinical investigators that the development of blood vessel and bone marrow masses may be a possible complication of stem cell therapy.
And they said more work was needed to identify why the masses formed, and how this could be avoided.
Writing in the journal, Andras Nagy, of Toronto's Mount Sinai Hospital, and Susan Quaggin, of the University of Toronto, said caution was needed over stem cell therapies - especially if they were being offered by unregulated private clinics.
They added: "Premature enthusiasm and protocols that are not fully vetted are dangerous and result in negative publicity for the field of stem cell research, and more importantly, may result in disastrous outcomes with no benefit to the patient.
"Although there is promise, a large gap still exists between scientific knowledge and clinical translation for safe and effective stem cell-based therapies.