Here is a summary of the laser's major medical uses: Ophthalmology

Lasers are used to treat most of the major causes of blindness. These include retinopathy, abnormal bleeding in the back of the eye; macular degeneration, bleeding in the macula -- the central part of the retina that is responsible for straight-ahead vision and sight used in reading or driving; and glaucoma, excessive fluid buildup inside the eyeball.

The argon laser beam is used to seal off leaky blood vessels in the retina in an effort to halt or control retinopathy, which eventually strikes at least one third of people with diabetes. Anywhere from 1,200 to 3,000 split-second pulses painlessly "spot-weld" the tiny retinal blood vessels. The same treatment is used on macular degeneration, the most common cause of vision loss in people over 60.

Studies sponsored by the National Eye Institute have demonstrated the effectiveness of lasers in reducing the chances of vision loss from both retinopathy and some types of macular degeneration, though treatment is not a guaranteed cure.

"The challenge now is to refine these treatments and determine when exactly they are most advantageous," said Dr. Robert P. Murphy, associate professor of ophthalmology at Johns Hopkins.

Lasers also treat glaucoma by making tiny holes in the iris and other parts of the front of the eye, to relieve the pressure in the eyeball. A decade ago, said Dr. Harry A. Quigley, director of glaucoma services at Johns Hopkins' Wilmer Institute, patients with angle glaucoma required intraocular surgery, with two weeks in the hospital and some risk of infection and bleeding in the eye.

"Today, we are into our second generation of lasers to do the same treatment," Quigley said. "The patient is treated as an outpatient, in both eyes, in 30 minutes, and goes home seeing nearly normally without eye patches."

Despite a widespread misconception, lasers are not used to remove cataracts from the eyes. But they are used to treat a clouding of the clear membrane behind the lens -- a condition that occurs within five years in about 30 percent of patients who have a cataract removed and an artificial lens implanted.

"We cut a little plus-sign with a laser in the middle of the pupil, and the tissue folds back out of the way, giving the patient a window to look through," said Dr. Douglas Gaasterland of Georgetown University Medical Center. "It's not painful, and the risk of bleeding and infection is almost nonexistent."

Use of lasers to improve vision by altering the shape of the cornea or making a series of tiny cuts in the cornea is under study. Dermatology and Plastic Surgery

One of the earliest uses of laser therapy was in removing purplish-red "port wine stain" birthmarks and other vascular lesions that result from an excess of blood vessels in the skin. In many cases, the argon laser effectively removes such lesions quickly and painlessly, with little scarring.

"When you destroy tissue, there's always the potential for scarring," said Dr. Max Ruben, a dermatologist in the District. "But if the laser is used properly, the scarring is very small, because the laser is so precise."

The laser is also used to remove tattoos and keloid scarring from the skin, though it doesn't necessarily prevent recurrences of keloid scarring.

Research is under way on use of lasers to treat skin cancer, though the evidence so far suggests little advantage over traditional therapy for most types of skin tumors. Otolaryngology (Ear-Nose-Throat)

Lasers are used to remove many types of benign and malignant growths in the mouth, throat, larynx, vocal cords and trachea.

In children, congenital abnormalities such as thin web-like membranes blocking the throat and buildups of blood vessels known as hemangiomas are treated by laser. Conventional surgery on hemangiomas is extremely difficult.

"After you took your first removal of tissue, there was blood all over the place," said Dr. Stanley Shapshay of the Lahey Clinic. "There went your precision. You couldn't see what you were doing."

A technique developed by Dr. David E. Fleischer, now at Georgetown, uses lasers to treat patients who cannot swallow because of a blockage of the esophagus caused by a gastrointestinal cancer. Though such treatment does not cure the cancer, it can greatly improve the patient's well-being.

"In some cases, we took people who could barely swallow liquids, and now they can eat solid food," Fleischer said. "It really changes the quality of life, and in some cases extends it."

A similar technique uses lasers to open an airway blocked by the spread of lung cancer. The YAG laser is beamed through a flexible, wire-thin scope threaded down the throat to the blocked airway, where it is aimed at the tumor and fired.

"It's not used as a knife, but as an infinitely sophisticated soldering iron," said Dr. Bernard Marsh, an otolaryngologist at Johns Hopkins. "If we tried to do that without a laser, we'd have blood all over the place."

"It doesn't cure the lung cancer," Marsh said, "but it extends quality of life -- often for many months -- for many patients who otherwise would have to go back to bed until they die." Gastroenterology

Lasers have two main uses in the gastrointestinal tract. They can help control bleeding from some ulcers and other lesions, and they can treat some cancers and other growths in the stomach and large intestines.

The YAG laser is the primary type used in gastroenterology, because it can be threaded through a flexible endoscope, along thin glass fibers to hard-to-reach areas of the gastrointestinal tract. Gynecology

One of the major surgical uses of lasers is in gynecology, because of the need for precision in removal of tissue without damaging surrounding tissues. The carbon dioxide laser is used to remove viral warts, as well as lesions of the cervix, vulva and vagina.

The laser is also used, usually in combination with radiation or drug therapy, to treat some kinds of ovarian cancer and some tumors that have spread to the female reproductive organs from other sites.

Infertility resulting from scarring or other damage to the uterus or Fallopian tubes is, in some cases, treatable with lasers. Excessive menstrual bleeding can also be stopped by YAG laser treatment, but usually with the loss of fertility. Cardiovascular

Lasers have had little role in treating heart disease, but researchers still hope to perfect a way to unclog diseased arteries with a beam of light.

One problem with experimental efforts so far is that laser-opened arteries tend to close up again within a few months. Another is the challenge of removing the plaque along a blood vessel wall without puncturing the wall itself.

"We've got a long way to go," said the Lahey Clinic's Shapshay. "It's okay for the larger blood vessels, such as in the leg, but it's not so easy in the coronaries."

Coronary arteries, the network of blood vessels supplying oxygen and other vital nutrients to the heart, are much narrower and more numerous than the large arteries in the leg.

"You can get the laser into the blood vessel," said Shapshay, "but how do you channel it around corners into the smaller branches of the vessels. How do you remove the plaque and not put a hole in the blood vessel?" Neurosurgery

Lasers are used in combination with conventional surgical instruments to remove some types of benign lesions in the brain.

The laser has two advantages in neurosurgery. It reduces bleeding, and its precision lessens the risk of damage to crucial nerves and blood vessels surrounding the treated area. Such damage could result in paralysis, blindness or loss of coordination. Urology

Lasers are used to treat infectious warts, known as condylomas, on the surface of the penis, and some tumors of the penis. They can also treat a type of bladder tumor called transition-cell carcinoma. Podiatry

Many podiatrists use lasers to treat a range of foot problems, including warts, keloids and other scars, ingrown toenails, calloused tissue within the skin under the heel and neuromas, benign growths that occur between the bones in the lower foot.