Tens of thousands of patients with multiple sclerosis (MS) may benefit from the treatment if the tests become successful, say the researchers.
The researcher admit that they cannot say in how many months or years their treatment will begin to undo the damage caused by the incurable disease that affects the central nervous system, or whether it would actually work or not.
They, however, are confident that the stem cell therapy will be a major breakthrough for the 85,000 people in the UK who suffer from MS, many of whom are left wheelchair-bound and paralysed.
"We believe that bone marrow cells have the capability to repair precisely the type of damage that we see in the brain and spinal cord in MS. So by giving patients very large numbers of their own bone marrow cells we hope that this will help stabilise the disease and bring about some repair," the Telegraph quoted Neil Scolding, professor of clinical neurosciences for North Bristol NHS Trust, who is leading the trial, as saying.
The trial started six months ago, and it involves six people with MS, aged between 30 and 60.
A pint of bone marrow is extracted from the patients' pelvises, and the process material containing stem cells is injected on the same day into their arms.
The patients will be monitored closely over a period of months, and will be given regular brain scans to see what impact the treatment has had on them.
The Frenchay trial avoids the ethical controversy that surrounds many stem cell studies, as human embryos are not being used in it.
Prof Scolding conceded that though the first patients in the trial underwent the stem cell therapy six months ago, it was yet unknown whether there had been any benefits or whether the MS sufferers would need more than one injection of stem cells.
"I'm hoping there will be some improvement," Liz Allison, an MS patient taking part in the trial, told the BBC.
"We're delighted that this new trial is going ahead and there will be an awful lot of people with MS watching it very closely," said Christine Jones, the chief executive of the MS Trust.