Patients who had human embryonic stem cells implanted into their eyes to correct vision loss have experienced no long-term ill-effects, a study has confirmed.
Up to three years after treatment, the procedure still appears to be safe, scientists reported.
There had been concerns about tumours being triggered by hyperactive stem cells and immune system rejection.
In more than half the patients, the treatment also restored some sight.
All 18 patients given the experimental therapy had different forms of macular degeneration, a common cause of blindness and impaired vision.
Prof Robert Lanza, chief scientific officer at the US company Advanced Cell Technology, which funded the research, said: "Embryonic stem cells have the potential to become any cell type in the body, but transplantation has been complicated by problems including the risk of tumour formation and immune rejection."
Human embryonic stem cells (hESCs) are blank-slate "mother cells" taken from early-stage embryos that have the ability to develop into any type of body tissue.
The study found no evidence of hyper-proliferation or rejection during a typical follow-up period of 22 months with the patients. Tests showed substantial improvement in 10 of 18 treated eyes. Eight patients were able to read more than 15 additional letters on a sight chart a year after treatment.