Stem cell therapy used on heart disease patients can make their coronary arteries become narrower, according to new research.

A study was abandoned when scientists discovered that although participants' heart function improved, there was an unexpectedly high rate of coronary artery narrowing.

The study was published in The Lancet.

The researchers, from Seoul University, South Korea, were carrying out a study on 27 patients who had been diagnosed with myocardial infarction.

Seven of the patients were randomly assigned to receive an infusion of blood stem-cells and a cell mobilising agent called granulocyte-colony stimulating factor (G-CSF).

Treatment showed improved cardiac function and promoted the growth of blood vessels in the patients.

But the authors said the effect of the narrowing of arteries 'could be a serious problem''.

They conclude: 'In future studies with G-CSF based stem-cell therapy, patients should be carefully monitored for unexpected effects.''

Reference: Hyun-Jae K et al (2004) Effects of intracoronary infusion of peripheral blood stem-cells mobilised with granulocyte-colony stimulating factor on left ventricular systolic function and restenosis after coronary stenting in myocardial infarction: the MAGIC cell randomised clinical trial Lancet 363 (9411) 751-756