Acne, America's most common skin disease, affects more than 16 million people and causes both visible and emotional scars. But over the last 10 years researchers have found new treatments so effective that virtually all acne -- even seemingly hopeless cases -- can be cured.

In recent years, researchers have exploded a number of acne myths, including the idea that it must be passively -- and sometimes painfully -- endured.

"Acne shouldn't be ignored because the treatments available . . . are effective," said Dr. Gary L. Peck, a National Cancer Institute expert on acne and other skin diseases.

No one breakthrough is responsible for the success in treating acne. Rather, it is an accumulation of treatments that have recently been introduced.

Most mild acne can be successfully treated with nonprescription acne soaps and benzoyl peroxide, a medicine that prevents the plugging of pores that leads to pimples, according to Peck.

In addition, Peck said, two kinds of prescription medication, retinoic acid cream and an antibiotic lotion, clear up many of the more severe cases without producing significant side effects.

For severe cystic acne, which sometimes defies other treatments, Peck has found that an experimental drug, another kind of retinoic acid taken as a pill, works in almost every case.

A nontoxic relative of Vitamin A, the drug, 13-cis-retinoic acid, will soon be considered for approval by the Food and Drug Administration. It may also prove effective for other skin diseases and some kinds of cancer.

(Vitamin A itself cannot be taken in high doses because of side effects on the liver and nervous system.)

Much about acne is still a mystery. Left to itself, acne does not always disappear with adulthood or even with the arrival of gray hair.In fact, no one knows why it ever goes away.

Acne is not caused by dirt, stress, chocolate or nasty thoughts, according to Peck. It cannot be cured by changing one's diet or getting more sleep. Above all, Peck said, it is not the patient's fault.

It is also not a disease of only teenagers.Even though it is most prevalent during adolescence -- affecting 86 percent of all 17-year-olds, according to a report prepared for the National Institutes of Health -- acne also afflicts significant numbers of adults. In fact, the most severe form -- cystic acne -- is most common in people between 18 and 34 years old, many of whom may have enjoyed clear skin when they were younger.

Americans' concern with acne is reflected in what they spend on it -- almost $200 million a year for doctors' visits and at least $125 million more for prescription and and nonprescription drugs. Acne patients make up almost 28 percent of dermatologists' practices.

Most of the money goes for cosmetics and cover-ups, which can sometimes irritate skin cells and make the disease worse.

Dermatologists seem to know more about what doesn't cause acne than what does. Two factors do play important roles. One is heredity. "If your parents had bad acne," Peck said, "your chances of having bad acne are excellent."

Another is hormone -- especially testosterone, which is known as the male hormone but which women also produce in small amounts. Acne is more common in boys, and is a sign of sexual maturation since it usually begins during puberty when hormone levels rise. "Eunuchs don't get acne," Peck said.

But many other suspected causes have been exonerated, even though they are still blamed by some doctors and parents.

Food is one. For years, teen-agers have been told their skin breaks out because they eat chocolate, peanut butter or junk food, and drink cola.

Not so, said Peck. "Diet doesn't seem to play a role," he said, "unless you eat a lot of sea kelp." (Sea kelp is rich in iodine, one chemical that does make acne worse.)

Dirt is another. Acne sufferers often hear that they must wash their faces more often so dirt won't clog their pores. Peck said he went to a Montgomery County public school five years ago to see a film about acne that was being shown to students.

"They actually zoomed in on a blackhead and said, 'See, that's dirt'," he recalled. "At the end of the movie, the children applauded."

Peck wrote a letter to the school, explaining that blackheads are not pores plugged by dirt, but by accumulated cells and skin pigment. He never received a reply.

He said there is also no evidence that stress plays a role in most acne, even though patients are often told to relax or get more sleep. The only exception he has seen is the occasional adult who, after a lifetime of clear skin, has an explosion of acne after an emotional event such as the death of a family member or the breakup of a marriage.

A pimple begins when cells lining one of the skin pores stick together to plug the pore instead of being shed as they wear out. This plugging happens more often when some hormones, like testosterone, are plentiful, and when others like estrogen, are scarce -- such as just before a woman's menstrual period. Cells may also form plugs when they are irritated by chemicals in cosmetics or by the products of bacteria.

Once the pore is plugged, sebum -- the oil made by skin glands -- cannot escape, and builds up behind the plug. Eventually, it ruptures the pore and leaks into surrounding issues, causing a focus of inflammation: a pimple or pustule. A pimple takes weeks to form and days or weeks to heal, and may leave a scar.

Benzoyl peroxide, which has been on the market for years and is an ingredient in many nonprescription acne remedies, works by producing extra shedding of skin cells, so that fewer plugs form. It also kills some skin bacteria. It can cause mild drying and peeling.

A newer remedy that works for more severe acne but requires a prescription is retinoic acid. Applied as a cream, it loosens connections between skin cells so that plugs are shed and new plugs do not form. It can produce temporary redness and peeling of the skin -- like a mild sunburn -- and may take two to three months to work, after which patients must still use it daily. t

The major possible side effect is that retinoic acid may make it easier for sunlight to damage the skin, thus increasing the long-term risk of skin cancer. Peck said this concern comes from one study done on mice, and needs to be confirmed, but most doctors caution patients not to use the cream if they expect to be outside a lot.

Antibiotics, especially tetracycline, have been used to treat acne for many years. Dermatologists believe they work by killing skin bacteria, which produce chemicals that encourage plug formation. But tetracycline sometimes has harmmful effects on the liver or bone marrow, and in women it can promote vaginal infections by killing the body's normal bacteria.

But Peck said that many patients can be treated now with equal success by antibiotic lotions, which are safer. He said a lotion containing the antibiotic, clindamycin, seems to have no adverse effects and can be used in combination with retinoic acid or benzoyl peroxide to clear up the average case of acne.

Person's suffering from severe cystic acne may have to wait another two years for the FDA to approve 13-cis-retinoic acid, Peck said. It is available now in only a few research centers, and Peck's study is accepting no new patients. Of the 90 patients he treated, 96 percent showed a clearing in their acne. The only side effect was dryness of the skin, eyes and mouth.