Scientists say they have found a drug which may repair brain damage caused by multiple sclerosis - a condition caused by damage to the protective sheath surrounding the central nervous system.
Researchers say the drug, alemtuzimab, restored lost brain function for some patients in the early stages of MS.
The drug was created at Cambridge in the late 1970s, and has long been used to treat leukaemia by killing off the cancerous white cells of the immune system.
It appears to stop progression of the multiple sclerosis in patients with early stage active relapsing-remitting MS - the most common form of the condition.
The University of Cambridge study, published in the New England Journal of Medicine, also suggests the drug may enable repair of previous damage.
However, it can produce potentially serious side-effects, they warn. And the researchers stress their work is still at an early stage.
The latest three-year study, of 334 patients with relapsing-remitting MS which had yet to be treated, found that the drug cut the number of attacks of disease by 74% more than the reduction achieved by conventional interferon-beta therapy.
Alemtuzumab also reduced the risk of sustained accumulation of disability by 71% compared to beta-interferon.
People on the trial who received the drug also recovered some function that had been thought to be permanently lost, and as a result were less disabled after three years than at the beginning of the study.
In contrast, people given beta-interferon showed signs of progressively worsening disability.
This was confirmed by brain scans in which alemtuzumab patients showed signs that their brains had actually increased in size, while the beta-interferon patients' brains shrank over time.