The incident occurred nearly 40 years ago, but the embarrassment is still etched clearly in Helen Vidnovic's memory.

"I was 17," she recalls, "and all dressed up for a special evening out. I was feeling wonderful until I overheard a neighbor whisper, 'Helen would be such an attractive girl if it wasn't for her chin.'"

Until that time Vidnovic hadn't been particularly self-conscious about the vibrant port-wine-colored birthmark. As a child she was teased, "but my parents were very positive about it, and I always thought I was kind of special with my rose-colored chin."

She'd become used to insensitive comments like, "Did your husband hit you on the chin?" or, "Did you fall on your face?" But as a realtor she found the birthmark detracted from her appearance, made people uncomfortable and hurt her business.

Several months ago, on a routine visit to her beauty salon, she talked with "esthetician" Tammy Wolfe, who told her about Covermark, a cosmetic created in the '30s by New Yorker Lydia O'Leary to cover her own disfiguring facial birthmark. Wolfe had recently introduced the special make-up in Lanham's Cut 'N Up salon. It is also available at Hecht's.

"Covermark is used to conceal scars, burns, skin grafts and other disfiguring marks that keep people from looking and feeling their best," says Wolfe. "We help post-operative patients, accident victims, Vietnam veterans and people with cogenital marks get back into the mainstream of life."

"It's an emotional thing," notes Vidnovic. "I feel better inside and out, and it helps me look and feel professional on the job."

Available in shades effective for any color skin -- from albino to black -- Covermark also is helpful in concealing the effect of vitiligo, which results in white, patchy pigment loss on dark skin.

Unlike most make-ups, "It will adhere to stretch marks and body scars where there are no pores," says Wolfe. It is moisture-resistant, she says, and won't come off when swimming, or from perspiration.

"We even have a gray shade for men who have skin grafts and want to fill in a spot where their beard won't grow. But I don't call it make-up with male customers. They're more comfortable if I call it pigment."

Some of Wolfe's clients are so sensitive about their disfiguring features that they come in through a back door, or visit the salon before or after regular business hours.

"If they're very sensitive I go to their homes," she says. "I worked with one woman who had acid burns on her face and hadn't been out of her home in four years except to go to church.

"She came in the back door, and I worked on her for several hours. When she turned around to look at herself in the mirror she cried. She left through the front door."

Since Wolfe began offering Covermark in September, she has visited numerous hospitals, burn centers and trauma units, explaining to health professionals the physical and psychological benefits of corrective cosmetics.

In June she is opening a Covermark Application Center -- one of about two dozen in the country -- to provide increased services and more privacy.

"Some people aren't comfortable going to a cosmetics counter or have a mark in an area -- like their thigh -- that isn't easily exposed in a store."

For Wolfe, Covermark application is an extension of the social-work profession for which she originally trained. When she couldn't find a social-work job with just an undergraduate degree in 1965, she worked on Capitol Hill and had a brief stint as a private detective.

She became an esthetician (a licensed skin-care specialist) in 1974 and learned of Covermark last fall after consulting a plastic surgeon about a burn she received from a defective heating pad.

"People get very depressed about surgical scars, marks or burns," says Wolfe, who has trained six operators in Covermark application. "We work with some people for four or five hours. It's a good feeling to help them."

This personal attention is one of the best features of Covermark, says Dr.

William Schorr of Wisconsin, a member of the American Academy of Dermatology's committee on therapeutic cosmetics.

"If properly done and taught it's an excellent product," he says. "The person may have a look of wearing too much make-up, but that's a lot better than having very unsightly marks."

Some physicians, like Bethesda plastic surgeon Dr. Bahman Teimourian, send patients to Wolfe as part of their treatment. "Plastic surgery can do a lot," he says, "But there are times when we've done as much as we can, and Tammy does a very good job showing people how to look their best."

Wolfe charges $25 for a one-hour application and lesson and $35 to clients referred by doctors who may need several hours of attention. Consultations are free. (The cost of a small container of Covermark is $7.)

"We ask that the surgeon include our work in his fee," says Wolfe, noting that some health insurance policies cover the cost. "But we never turn anyone away if they can't pay."