In what's being called a landmark study, researchers used gene therapy to successfully treat six patients with severe hemophilia, a blood-clotting disorder.

The study was preliminary and involved only six patients. Other promising early attempts to use gene therapy against hemophilia ultimately failed. But a single infusion using the new treatment worked in some patients for more than a year, boosting their clotting ability significantly.

"I think this is a terrific advance for the field. It's a good lesson in terms of don't give up on good ideas," says Dr Ronald Crystal, chairman of genetic medicine at New York City's Weill Cornell Medical College.

It's "truly a landmark study", says Dr Katherine Ponder, a Washington University School of Medicine physician. She praises the research in an editorial that accompanies the study's publication in the New England Journal of Medicine.

The research was also presented recently at an American Society of Hematology conference in San Diego.

Hemophilia is an inherited, potentially life-threatening disorder affecting mostly males. Their blood doesn't clot properly because of a faulty gene.

In severe cases, they can spontaneously start bleeding internally, even in the brain. Internal bleeding in the joints leads to debilitating movement problems and intense pain.

Past gene therapy experiments on hemophiliacs improved blood clotting for only a few weeks. "We couldn't make it last," says Val Bias, chief executive of the National Hemophilia Foundation.

Experts say the new method needs to be tested on many more patients to confirm whether it's effective and prove there are no risks. Even if all goes right, it's still several years away from being available to most patients.

Since the late 1960s, doctors have given hemophiliacs infusions of clotting proteins. It's been a success, increasing their average lifespan to 63.

But for severe cases, treatment can involve two or three infusions every week and cost more than $250,000 annually.

The new study was led by researchers at the University College London Cancer Institute and St. Jude Children's Research Hospital in Memphis, Tennessee. All six of the patients in the study were men seen in London who had severe forms of type B hemophilia.

The six men each received a single, 20-minute infusion of healthy genetic material delivered by a virus found in monkeys. Viruses are often used to transport DNA into cells. Each saw the amount of clotting proteins in their blood increase from less than 1 percent of normal levels to at least 2 percent, and in one case as much as 11 percent.

That may not seem like a lot, but it was enough to allow all the men to ease back on the number of regular treatments they needed, and four stopped conventional treatment altogether.

It's not yet clear how risky the treatment is. In one patient, the level of liver enzymes shots up to five times normal levels. It did not cause symptoms but there are concerns about liver inflammation and he was treated with steroids. Researchers note it's possible the treatment could trigger hepatitis in some patients.