Call it vanity or simply wanting to look good, an increasing number of men (and some women) are trying to hide their baldness, many with transplants.

Not that baldness is all that bad. "It's not a disease," says dermatologist and author Herbert S. Feinberg. One reasonable way to deal with baldness is "ignore it."

Still, millions of people haven't ignored their thin locks. Sen. William Proxmire (D-Wis.) had a much-publicized hair transplant seven years ago. Sen. Strom Thurmond (R-S.C.) had his five years ago. Frank Sinatra got one, and so did 32-year-old rock entertainer Elton John, who flew regularly from London to Paris for the long, drawnout surgical process.

"I admit it's 100 percent vanity, but I'm thrilled with the result," John said last year after his thinning hairline was covered-for $2,000-with a denser growth.

Actor Burt Reynolds, who started out with a hairpiece, is reported to be replacing it with an $8,000 transplant after hearing one too many gags about the fake.

But the names aren't all famous. The people who cover those glistening domes say their clients include men and women from all backgrounds, policemen and construction workers, doctors, lawyers.

The stigma is disappearing, says Feinberg, author of "All About Hair" (Simon and Schuster, 179 pages, $10). He credits the trend to Proxmire, who "bared his scalp to the world."

Feinberg estimates a million men and women have had transplants in the last 20 years. Significantly, he adds, at least half have been done in the last five. Feinberg performs six to eight transplants a week at his Englewood N.J., office.One out of 15 of his patients is a woman.

Says a 42-year-old Washington office worker who has been undergoing transplants here since June: "I just don't like being a bald." So far he's satisfied, and the procedure has been painless. Thurmond is satisfied, too, according to his press aide. Proxmire no longer speaks publicly about his transplant, but he doesn't hesitate to offer the name of the doctor who performed it.

By age 25, about 10 percent of American men have recognizable baldness and by age 65, about 65 percent, according to Feinberg (who at 42 has an extraordinarily thick growth of graying black hair, all his own).

Often it's a young man in his 20s with a hairline that's just begun to recede who shows up in the office of California transplant pioneer Samuel Ayres III. The young man is upset and he wants help now, says Ayres, whose exclusive Beverly Hills clientele has included Sinatra and Joey Bishop. Ayres counsels caution.

He paints a sad picture of the man with only a balding upper forehead who gets it covered with a transplant-only to lose the rest of his hair in the next few years. The trasplant is left high, dry-and noticeable.

In a typical transplant, small "plugs" of hair-bearing scalp are removed from the fringe hair behind the ears and transferred to the bald scalp. Each plug contains about 15 hairs, and depending on the expanse to be covered, the recipient may require up to 500 or more plugs (Proxmire had 200.)

Forty or more plugs may be transferred in one session, with a waiting period of two to four weeks before additional plugs are added. You go home with a bandage on your head for 24 to 48 hours. Initially, the transplanted hair falls out, but new hair should begin growing in about three months.

"Remember," says Feinberg, "We're not putting in sod. We're planting seed."

In Washington the cost of each plug ranges from $10 to $20, according to Dr. Donald Cameron, the Bethesda plastic surgeon who performed Proxmire's transplant. (For 200 plugs, that means $2,000 to $4,000.) Feinberg charges $15 per plug. In Beverly Hills, it's $50.

Despite its expense, the transplant is popular because it provides the balding with what they want most-more of their own hair growing up top. And, according to Cameron, the success rate when done carefully "approximates 100 percent."

A new "flap" technique of transplant involves cutting a patch of scalp from above the ears and folding it over the bald area to create a new hairline. If you are considering a transplant, here are some remainders:

Don't expect a transplant to give you the head of hair you had at 16. It can't be done.

Get your physician to recommend a dermatologist, plastic surgeon or other doctor well trained and experienced in transplants who does all the work himself.

Like it or not, not everyone is a candidate for a transplant. You may not have enough hair left to work with.

Think first about a good hair stylist to rearrange any locks that are left.

If a transplant is too expensive, consider a wig or toupee. As in many things, the better the hairpiece the more expensive.

Few of us have forgotten the jokes about toupees tumbling into the soup. New adhesives have been developed, feinberg says, to make that unlikely. Hair-replacement firms offer such techniques as hair weaves, hair fusion and suture implants to attach the hairpiece to the scalp.

One method of hiding baldness that has shown up in the United States recently is causing the Food and Drug Administration a great deal of concern-the implantion of hundreds or even thousands of synthetic hair fibers in the scalp. The FDA has received 125 documented complaints from around the country this year of scarring, pitting of the scalp, breaking of the fibers and serious infection from such implants.

Some men who have undergone fiber implants "literally have had to be surgically scalped" to have the fibers removed and plastic surgery performed to repair the damage, says Steven Niedelman, and FDA consumer safety officer.

About 30 firms nationally have performed or are performing fiber implants, Niedelman says. One Washington-area man wanted to return to a fiber implant clinic here last month to complain of infection. The clinic had closed.

A nationwide FDA investigation is under way, says Niedelman, and FDA is seeking regulatory action against fiber implants. Many firms are closing because of bad publicity, he says. CAPTION: Pictures 1 and 2, BEFORE and AFTER, Sen. Strom Thurmond; Pictures 3 and 4, BEFORE and AFTER, Elton John; Pictures 5 and 6, BEFORE and AFTER Sen. William Proxmire