A painless treatment for hemorrhoids that takes just a few minutes and doesn't leave the patient sore? It sounds too good to be true. But, in fact, that's what typically happens when an instrument called the infrared probe coagulator -- recently approved for the purpose by the Food and Drug Administration -- is used to treat hemorrhoids, distressing clusters of swollen blood vessels, supportive tissue and overlying skin in the anus or the mucous membrane of the rectum.

Made in Germany and widely used in Europe, the probe, which looks like a pistol, emits bursts of infrared light when the trigger is pulled. The light causes the blood within the knot of hemorrhoidal tissue to clot. Starved of a nourishing blood flow, the hemorrhoid shrinks and dies. The small burn on the lining of the anus or rectum created by the treatment heals in about a week, leaving no scar to cause possible trouble later on.

Dr. John J. O'Connor, a George Washington University colorectal surgeon, tested the device for the FDA and after using the probe in his office on more than 700 hemorrhoid sufferers since 1982 and thinks it works well.

"Sooner or later, a patient of mine may bleed," he said, "but it hasn't happened yet. Nor have any of my patients had their hemorrhoids recur."

Hemorrhoids, also known as piles, are named for the Latin word for ball, since they resemble a ball of disorganized blood vessels. Physicians normally rate the severity of the hemorrhoid in three degrees.

First-degree hemorrhoids stay strictly inside the rectum, the area above the anus which opens to the outside. Because the nerve fibers in the rectum are insensitive to pain and itching, first-degree hemorrhoids usually announce themselves only by bright red bleeding now and then.

Second-degree hemorrhoids also bleed occasionally. However, they come to the verge of the anus which, unlike the rectum, is richly supplied with nerves that register pain and itching. They are likely to be somewhat uncomfortable when inflamed.

More uncomfortable still and even more likely to bleed are the third type. They pop out of the anus during bowel movements, sometimes returning to their original location spontaneously and sometimes when the sufferer applies gentle finger pressure to push them back in.

Various methods have traditionally been used to bring these three types of hemorrhoids under control. Among the most popular are chemical injections into the offending blood vessels to reduce their size, tying off the trouble spots with rubber bands, burning them with electrocautery, or freezing them into submission with liquid nitrogen.

According to a study done in Ireland, the rubber band method causes more pain during and after treatment than the infrared probe and is no more effective. And in O'Connor's experience, at least, the same goes for the other older alternatives.

Furthermore, he found the new probe to be more reliable. For example, he said, the rubber band treatment may require several office visits, which is a nuisance for the patient and more expensive, too. In addition, though it doesn't happen often, all the conventional therapies performed in a doctor's office can produce so much bleeding that only major surgery will bring it to a stop.

Some bleeding hemorrhoids, however, are chronically external and extremely painful because that area -- primarily covered by skin, rather than mucous membranes -- is richly supplied with superficial nerves that convey itching, burning and pain. Sometimes as large as golf balls, these Type 4 hemorrhoids, as well some third-degree hemorrhoids, call for treatments more drastic than therapies such as the rubber band or infrared probe treatments.

In Britain, such tough cases are often successfully treated by anesthetizing the patient and stretching the tissues of the lower rectum. But in this country, until recently, nothing has been available except hemorrhoidectomy, an operation in which the diseased veins are tied off and cut out.

Surgery usually produces a lasting cure since new hemorrhoids rarely form after the operation. But surgery and recovery can be painful.

While the infrared probe may be good for mild hemorrhoids, the wave of the future for severe and persistent hemorrhoids may well be a type of laser therapy approved by the FDA last spring.

Pioneered by Dr. Howard Eddy and his colleagues, who practice both on Long Island and in Boston, it permits patients to go home an hour after the procedure confident that they will have little discomfort if they follow the instructions for taking care of themselves after therapy. This home care includes warm baths, special diets, the use of ointments, and allowing the treated area to heal.

However, unlike patients treated with the infrared probe, those getting laser treatment have to be away from work for at least 10 days and sometimes more. Many people get hemorrhoids in the first place because of the strenuous physical activity their job requires or because of prolonged enforced sitting, making it especially important that their tissues heal completely before they go back to work.