The operation involved inserting a gene into the eye, a treatment that revived light-detecting cells.
The doctors involved believe that the treatment could in time be used to treat common forms of blindness.
Prof Robert MacLaren, the surgeon who led the research, said he was "absolutely delighted" at the outcome.
"We really couldn't have asked for a better result," he said.
BBC News exclusively reported on the start of the trial two years ago. The first patient was Jonathan Wyatt, who was 63 at the time.
Mr Wyatt has a genetic condition known as choroideremia, which results in the light-detecting cells at the back of the eye gradually dying.
Improved visionMr Wyatt was still just about able to see when he had the operation. His hope was that the procedure would stop further deterioration and save what little sight he had left.
He, like another patient in Professor MacLaren's trial, found that not only did the operation stabilise his vision - it improved it. The other subjects, who were at earlier stages in their vision, experienced improvements in their ability to see at night.
Mr Wyatt is now able to read three lines further down in an optician's sight chart.
"I felt that I had come to the edge of an abyss," he told BBC News.
"I looked down at total blackness. Professor MacLaren tapped me on the shoulder and said 'come this way, it's possible to see again'."
Mr Wyatt's wife, Diana, confirmed that the prospect of total blindness had made him feel very depressed.
"Now he is very optimistic," she explained.
"He is more independent, he can find things he couldn't before, he can go to the shops on his own and he's less of a nuisance!"
Another of the patients who underwent the treatment, Wayne Thompson, said he had noticed an immediate effect after the operation.
Seeing stars"My colour vision improved. Trees and flowers seemed much more vivid and I was able to see stars for the first time since I was 17 when my vision began to deteriorate," he told BBC News.
Mr Thomson said he had spent his life resigned to the fact that he would go blind.
"I've lived the last 25 years with the certainty that I am going to go blind and now (after the operation) there is the possibility that I will hang on to my sight," he said.
When Mr Thompson was first diagnosed, he was told that he would not be able to see his daughter, who is now nine, grow up.
"Now I hope I'll see my grandchildren grow up," he told BBC News.
If the improvements seen in the patients continue, the aim will be to offer the treatment to younger choroideremia patients to prevent them from losing their sight.
The condition is relatively rare: it is thought to affect a thousand people in the UK.
But Professor MacLaren believes that success with choroideremia demonstrates the principle that gene therapy could be used to cure other forms of genetic blindness including age-related macular degeneration.
This condition causes blindness in 300,000 people in Britain and causes a deterioration in the vision of one in four people over the age of 75.
"The mechanisms of choroideremia and what we are trying to do with the treatment would broadly be applicable to more common causes of blindness," the professor explained.
"Choroideremia shows some similarities with macular degeneration in that we are targeting the same cells. We don't yet know which genes to target for macular degeneration but we do know now how to do it and how to put the genes back in."
Clara Aglen of the Royal National Institute of Blind People is also cautiously optimistic.
She told BBC News: "It is at an early stage at the moment, but it does offer hope for other conditions that have a genetic basis such as macular degeneration and glaucoma.
"As this process advances there is hope that it could be transferred across and provide a cure for these common causes of blindness."
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