A radical treatment that wipes out and then regenerates the immune system can halt progression of aggressive multiple sclerosis and even reverse its symptoms, research has shown.
Scientists in Canada described results from a trial involving 24 patients with a highly active, relapsing form of the autoimmune disease as "very exciting".
But at the same time they warned that the procedure was risky, and only likely to benefit a certain proportion of patients still in early stages of the illness.
MS occurs when the immune system strips nerve fibres in the brain and spinal cord of myelin, a fatty insulating material without which nerve signals cannot be transmitted properly.
Symptoms range from blurred vision or tingling sensations to full blown paralysis. Usually the disease progresses to become more aggressive over time, but some patients find their health deteriorating rapidly.
Doctors testing the therapy, known as IAHSCT (immunoablation and autologous hematopoietic stem cell transplantation), took stem cells from patients' bone marrow and froze them before injecting powerful chemotherapy drugs to destroy the immune system.
The stem cells were then transplanted back into the body to generate a new functional immune system with no "memory" of attacking the brain.
Results over a period of up to 13 years were dramatic, with not one patient relapsing and 70% experiencing a complete halt in disease progression.
In 40% of cases, patients saw lasting reversal of symptoms such as vision loss, muscle weakness and balance problems, the scientists reported in The Lancet medical journal.
Some participants were able to return to work or school, regain the ability to drive, or get married and have children.
Dr Harold Atkins, from the University of Ottawa in Canada, said: "Our trial is the first to show the complete, long-term suppression of all inflammatory activity in people with MS.
"This is very exciting. However, it is important to note that this therapy can have serious side effects and risks, and would only be appropriate for a small proportion of people with very active MS. People with MS who have had significant disability for a long time would likely not benefit."
A similar procedure has been used for decades to treat patients with life-threatening leukaemia.
They face similar risks from the severe side effects of the drugs used and the threat of infection while unprotected by an immune system.
One participant in the study died of liver failure due to the treatment and another required intensive care for liver complications. All the patients developed fevers which were frequently associated with infections.
Trial volunteer Jennifer Molson, who was diagnosed with MS in 1996 at the age of 21 and received the treatment in 2002, said: "Before my transplant, I was unable to talk or work and was living in assisted care.
"Now, I am able to walk independently, live in my own home and work full time. I was also able to get married, walk down the aisle with my dad and dance with my husband. I've even gone downhill skiing.
"Thanks to this research I have been given a second chance at life."
The trial, which cost 6.47 million Canadian dollars (£3.5 million), was funded by the MS Society of Canada, whose chief executive Yves Savoie said: "What started as a bold idea has translated into a treatment option for people living with highly active, relapsing MS.
"Publication of the results from this study will inform clinicians of the risks and benefits of the procedure, and pave the way for further research which could help people with all forms of MS."
Dr Emma Gray, head of clinical trials at the MS Society in the UK, said: "This type of stem cell transplantation is a rapidly evolving area of MS research that holds a lot of promise for people with certain types of MS.
"In this latest trial patients were monitored post treatment for a longer period than previous studies, providing valuable information about the long term safety and effectiveness of HSCT as well as who might benefit.
Professor Siddharthan Chandran, from the Medical Research Council (MRC) Centre for Regenerative Medicine, University of Edinburgh, said: "This is an important and carefully conducted proof of concept study that demonstrates that powerful chemotherapy-based treatment for a selected subset of MS patients with very aggressive disease is effective in preventing further disabling relapses and, in a proportion, appears to render them effectively disease-free.
"However, the treatment regime has substantial risks and safety concerns that underline the need for future studies with a larger sample size, control group and ideally identification of predictive markers to allow targeting of this treatment to those MS patients at greatest risk of rapid and severe decline."
Ms Molson is from Ottawa, Canada.