A stem cell treatment for heart attack patients has shown some promise but a high complication rate, say scientists.
The treatment uses stem cells found in the blood
Researchers from Seoul National University Hospital, Korea, stopped recruiting patients for their study after seeing the early results.
They said the treatment did appear to improve how the heart functioned, and to help new blood vessels develop.
But writing in The Lancet, they said much more research was required into the complications they had seen.
Most stem cells are found in the bone marrow, but this treatment uses stem cells called peripheral blood stem cells (PBSCs) which are found in a patient's bloodstream.
A chemical called G-CSF can also be used to boost production of the stem cells.
The Korean scientists were investigating whether the technique can be used to treat heart attack patients because collecting bone marrow cells for treatment is highly invasive.
They studied patients who were stable after having a heart attack.
All underwent coronary stenting - where narrow tubes are inserted to combat the narrowing of arteries.
The patients were all given G-CSF. Some patients were also given an injection of stem cells - which had been harvested from their blood prior to any other treatment - directly into their hearts.
After 11 patients had been followed up for six months, it was found the seven who received stem cells plus G-CSF had improved heart function.
But five of the seven, and two of three who received G-CSF alone, experienced in-stent restenosis - where coronary artery narrows again, despite the stent treatment.
After researchers saw these preliminary results, they stopped recruiting patients to take part in the study.
Writing in The Lancet, the researchers, led by Dr Hyun-Jae Kang, said: "Despite the favourable effects on cardiac function, our data warrant a more cautious approach to stem cell therapy, in view of the possible aggravation of restenosis."
In a commentary in the same journal, Dr Hiroaki Matsuba of the Kyoto University Hospital in Japan added that potential complications should be thoroughly investigated before such methods were more widely used in patients.