An experimental stroke treatment that sends laser beams into the brain may help patients with milder strokes lessen their crippling effects, U.S. researchers said on Friday.

Carlsbad, California-based PhotoThera Inc's NeuroThera Laser System failed to significantly lower stroke disability overall in 660 stroke patients in its first major clinical trial, the researchers wrote in the journal Stroke.

But patients with moderate to moderately severe strokes did improve after laser treatment, the researchers said.

Stroke is the No. 3 cause of death in the United States and a leading cause of disability, and experts say new stroke treatments are urgently needed.

The only treatment approved by the Food and Drug Administration for the most common type of stroke, caused by a blood clot, is a clot-busting drug known as tissue plasminogen activator, or tPA, that must be given intravenously within three hours.

"We need something for patients who come in later than that," Dr. Justin Zivin of the University of California San Diego, who led the study funded by PhotoThera, said in a telephone interview.

Doctors used the laser device to painlessly send low-energy infrared radiation into the brain, on average 14-1/2 hours after a stroke.

Ninety days later, 36 percent of those given the laser treatment had either mild or no disability compared to 31 percent of those not given the therapy, but the difference was not statistically significant, the researchers said.

Among those who had moderate to moderately severe strokes, 52 percent of those given the laser treatment had either mild or no disability compared to 42 percent of the others. That difference was statistically significant, Zivin said.

"We're very encouraged by it," added Zivin, whose study was presented at an American Stroke Association meeting.

Zivin, who has a financial stake in the company, said it is unclear exactly how the laser treatment may be helping the patient. It's possible the treatment is reducing cell death, increasing new cell growth and boosting energy metabolism.