Eye specialists at London's Moorfields Eye Hospital in Britain have restored a man's sight by using gene therapy.
The man, 17-year-old Stephen Howarth suffers from Leber's Congenital Amaurosis which is caused by a fault in a single gene, and leads to a progressive loss of sight.
The gene therapy regenerated the dying cells in the eye in the third of such operations by the team.
The operation involves injecting fluid containing the missing gene within a modified virus into the eye
Within a few months, doctors detected some improvements but Stephen himself did not notice these changes until he confidently walked through a dimly-lit maze designed to test his vision and surprised his doctors with the level of his improvement.
Prior to the therapy Stephen had only been able to see the bright lights of passing cars, street lamps and brightly-lit buildings now for the first time he has the confidence to walk out at night and socialise.
Without the operation it was likely that Stephen would have lost his sight altogether.
While the gene therapy has not improved the vision of the other two patients it may well stop their vision from declining further.
The successful result has wide implications as it establishes that gene therapy can work and suffering can be alleviated; it could also lead to treatments for several other conditions, including blood and immune disorders.
Inherited retinal degeneration affects 1 in 3,000 of the world's population and the success of the research offers hope to millions affected by eye diseases.
In this groundbreaking experiment the scientists say they were cautious and deliberately used a low dose and tested it on a patient suffering from a late stage of Leber's disease.
Professor Robin Ali of human molecular genetics at the University College London's Institute of Ophthalmology, says in younger patients an increased dose is expected to deliver even better results.
The researchers believe their technique could be used to treat a wide variety of sight disorders, possibly even age-related sight loss.
The research which was funded by the Department of Health, is published in the current edition of the New England Journal of Medicine.