RALEIGH, N.C. — Millions of Americans have undergone laser eye surgery to correct bad vision, and along with the procedure's popularity something else is coming into focus: its hazards.

Advertising stresses the surgery's safety, and most procedures are successful. Tiger Woods, who relies on keen eyesight as the world's best golfer, pitches it as a quick and painless way to restore sharp vision. Even the U.S. Air Force, long skeptical of the surgery, changed its policy last May to let people who had LASIK apply for pilot training.

But every year thousands of Americans who undergo LASIK are left with chronic pain, dryness of the eyes, distorted night vision and even blindness, according to Food and Drug Administration statistics.

LASIK — which stands for laser-assisted in situ keratomileusis — uses lasers to cut and reshape the cornea. It can improve eyesight without complications, but equipment flaws, a surgeon's error or a failure to screen out patients whose eyes are ill-suited for the treatment can cause the operation to go awry.

The American Society of Cataract and Refractive Surgery, which represents about 9,000 ophthalmologists specializing in laser eye surgery, estimates that only 2 percent to 3 percent of the more than 1 million LASIK surgeries each year are unsuccessful. But FDA records of clinical studies show that six months after the surgery, up to 28 percent of patients complained of eye dryness, up to 16 percent had blurry vision and up to 18 percent had difficulty driving at night.

The Triangle, home to two medical schools, is a hot spot for LASIK, where 11 eye centers will perform LASIK on about 8,000 patients this year, according to market research.

One of the leaders is Duke Eye Center, whose LASIK surgeons are among the best-trained and best-equipped in the field. But even surgery at Duke's level has damaged a few patients' eyes beyond repair.

One of those patients is Matthew Kotsovolos, 38, of Raleigh. He had more reason than most patients to feel confident about undergoing LASIK. He was the Duke Eye Center's head of finances. As an employee, he said, he was promised "red carpet treatment" and the procedure would be free.

The surgery June 8, 2006, gave him 20-20 vision, but it left him with intensely dry eyes and excruciating facial pain. He wakes up with sore eyes every morning, puts on special goggles to preserve eye moisture and wonders when the pain in his face will start to kick in.

"I traded in my glasses for permanent head pain, eye pain and these things," Kotsovolos said, pointing to the goggles.