Spark’s product, called SPK-RPE65, is aimed at retinal diseases caused by mutations in a gene called RPE65; this gene plays a role in maintaining the health of the photoreceptors in the eye. Spark estimates there are 3,500 people in the United States and five major European countries with these conditions, though the treatment would not be expected to help people whose disease had progressed past a certain point.

The 31 patients in the study, ranging in age from 4 to 44, have one type of Leber’s congenital amaurosis, which causes night blindness and an erosion of peripheral vision, and can eventually lead to total blindness.

Twenty-one of the participants were randomly assigned to have a virus carrying the RPE65 gene implanted into their eye via a surgical procedure. The 10 others, the control group, received no treatment.

The main measure of effectiveness was how much light participants needed to successfully navigate an obstacle course of sorts — following black arrows on a white tile floor, going up and down steps, and avoiding objects like wastebaskets. There were seven possible levels of illumination ranging from that of a moonless summer night to that of a brightly lit office.

One year later, those who had received the treatment improved by an average of two light levels, meaning they could complete the course in dimmer light than before, Dr. High said. That was better than those in the control group by a statistically significant amount, though she declined to provide the results for the control group. Two-thirds of those in the treated group were able to complete the course in the dimmest light, the level corresponding to a moonless summer night.

The company said there were no serious side effects or immune system reactions in the trial.

“It’s very exciting,” said Gordon Gund, chairman of the Foundation Fighting Blindness, which helped pay for the development of the therapy and some earlier studies of the therapy. He said that while the condition Spark is treating is rare, “this really provides us a platform for many other successful gene therapies.”

One question is how long the effect will last. Theoretically, gene therapy could provide a permanent fix. (Spark claimed the ticker symbol “ONCE” when it went public early this year.)