Jennifer Moore had been "very, very conscious" of her bust size for years, and this summer the 24-year-old decided to do something about it. It cost her $6,000 and a few days of pain and swelling, but the woman who was a 32A is now a 34C, thanks to her new breast implants.

"I just love how it looks, and my boyfriend really does, too," said Moore, a sales clerk from Frederick. "My mom said that if she was my age again, she'd do it, too."

The number of women electing to have their breasts enlarged through implant surgery is increasing rapidly. A record 220,000 American women chose to undergo breast augmentation last year, and the industry projects an almost 10 percent increase this year.

That is twice the number of women who were getting cosmetic breast implants a decade ago, before the Food and Drug Administration strictly limited use of the most popular type of implant -- the kind filled with silicone gel -- after reports that it might cause debilitating illnesses.

At first slowly, and now quite eagerly, many American women have turned to the saltwater-filled alternative to silicone implants. The two breast implant manufacturers in the United States recently reported record sales and profits for their spring quarters, and cosmetic plastic surgeons say the operation has reached a level of social acceptance unimaginable not many years ago. And not only are more women choosing implants, but they are choosing ever-larger models -- from an average of 250 cubic centimeters in the 1980s to about 350 cubic centimeters today.

Even concerns about silicone implants have eased significantly, and one manufacturer is expected to ask the FDA later this year to approve them for general use once more.

But some public health advocates and physicians remain alarmed about implants of all types -- especially now, with their resurgent popularity. Additional research, they say, has confirmed that planting a device in a woman's breast can cause serious, predictable and often costly complications, and they say the FDA is not providing American women the information and protection they need.

The most recent data presented to the FDA showed, for instance, that almost one-quarter of all cosmetic saline, or saltwater-filled, breast implants will need to be followed by another operation within five years, and that few implants can be expected to last more than 10 years. Studies have also found significant levels of internal infection, hardening of the tissue around the implanted device and implant leakage and deflation.

"This is a cosmetic operation with serious health consequences, and the FDA is just not treating it with the seriousness it requires," said Diana Zuckerman, president of the National Center for Policy Research for Women & Families and a longtime critic of the breast implant industry. "The benefits are so small compared to the very real risks, so it should be getting more scrutiny, not less."

FDA officials say the agency has spent years reviewing breast implants and allows them on the market because they meet the standards for safety and effectiveness. They also say it is important that implants are available to women who have lost breasts to cancer; they accounted for an additional 80,000 implants last year.

The popularity of cosmetic breast implants is taking off with questions remaining about the largest implant manufacturer, Mentor Corp. of Santa Barbara, Calif. A long-standing criminal investigation has focused on Mentor's research procedures and allegations of document destruction and faulty manufacturing. In addition, at an FDA hearing in July, the company was sharply criticized by experts for the small number of women it was able to contact for an FDA-required follow-up study.

The potential problems at Mentor are receiving renewed attention because a former compliance officer with the FDA has gone public with what he says were his long efforts to push the agency to redress problems at the company. James Austin Templer, who was the FDA official in charge of overseeing a Mentor plant in Texas in the mid- and late-1990s, has said that the company was in violation of good manufacturing and record-keeping rules, and that the FDA has not been sufficiently aggressive in its oversight. Templer quit the agency in protest in 2000.

"I was very concerned about the products that Mentor was putting out," Templer said in a deposition last month. "And I felt that the agency had just not done a good job with handling it."

The FDA has declined to comment on Templer's charges. Mentor said in a recent filing that it had no contact with federal investigators since early 2001, and that its research and manufacturing are regularly reviewed and approved by the FDA.

The willingness of many women to undergo surgery to gain larger breasts indicates that many American women are unhappy with the way their bodies look, according to David Sarwer, a psychologist at the University of Pennsylvania School of Medicine.

His research, consistent with statistics collected by the American Society of Plastic Surgeons, shows that although many women are getting implants in their twenties, as many are getting them in their thirties or forties, after they have had children. A surprising number -- more than 5,000 in the past two years -- are 18 or younger.

"The bar has gotten higher and higher for positive body images," Sarwer said. "We're seeing increases in all forms of cosmetic surgery as women try to keep up with the images they see all around them. Instead of putting their money into a 401(k) or going on a vacation, many women see this kind of surgery as an investment in themselves." Most, he found, are dissatisfied with how they look but are not more prone to depression or other psychological problems than other women.

The new acceptance of breast implants is remarkable, given the nation's response a decade ago to reports of problems with silicone breast implants.

Research linking silicone with a class of often-debilitating illnesses known as connective-tissue disease caused a sensation then, and led to class action lawsuits and big (though still unresolved) cash settlements. Silicone gel implants were limited to women who needed breast reconstruction after surgery or who would take part in FDA-reviewed clinical studies.

But the agency was sharply criticized by women who wanted implants and by plastic surgeons who lost a profitable business that they believed was safe and useful. An Institute of Medicine report in 1999 concluded there was unlikely to be a correlation between silicone implants and connective-tissue disorders, and to many, that settled the question. Since then, the FDA has struggled in this charged atmosphere to regulate implants.

Since the agency gave its retroactive approval to saline devices in 2000, it has reported that many women are satisfied with their implants and that the implants help them feel better about themselves. (Implant manufacturers say their studies show that many of the follow-up operations are performed for women who want even larger implants.) But the FDA also says that some health problems are inevitable, and that implants can make it more difficult to detect breast cancer; can cause numbness in the nipple; and can make it difficult or impossible to breast-feed an infant.

The experiences of Lisa Marie Channing of Arlington, who wanted implants after she lost weight and was unhappy with how her breasts looked, show some of the possible pitfalls.

The saline implants put in by her board-certified plastic surgeon in 1998 were bigger than she expected -- a double D, she said -- and they became hard and painful. After two months, she was so upset that she asked to have them removed. She said her surgeon initially balked but later acquiesced. The surgery, however, left visible scars on her breasts, she said. She has spent the past several years trying to have her breasts restored.

"I really didn't know what I was getting into, and now I'm disfigured," said Channing, 29. "My doctor told me it was all perfectly safe, but it wasn't and has caused me incredible problems."

While the average breast augmentation costs $5,000, Channing said she has paid much more to have her breasts repaired. Since cosmetic breast enlargement is not covered by insurance, as reconstructive surgery is, women pay for the surgery and any future operations -- work that earned plastic surgeons $669 million last year. As the number of cosmetic implants has skyrocketed, the number of expensive breast implant removals has also increased, to 42,000 last year alone.

While pain and hardening from the scar tissue around the implants are common, the biggest medical issues involve implant deflations. The leakage of saltwater is generally not considered dangerous -- though it can contribute to infections. The health risks from leaking silicone implants remain controversial. Research by the FDA and National Institutes of Health has recently found increased fibromyalgia (a disease of generalized pain and tenderness) in women with silicone implants, as well as an unexplained increase in brain cancer.

Plastic surgeons insist that women are made aware of the possible complications and say they require a signed "informed consent" form before implant surgery. But online chats with women who have had bad experiences with implants are filled with stories of unaware patients. And Leroy Young, a St. Louis doctor who is a breast augmentation expert for the American Society of Plastic Surgeons, said that in his experience, many women don't take the warnings seriously. "Women see this mostly as a legal thing, since the actual results are so good," he said.

Unlike the experience of a decade ago, the problems with implants today have drawn little attention. The breast implant debate is likely to heat up soon, however.

Officials at Inamed Corp., also of Santa Barbara, have said they are finishing clinical studies of silicone gel implants and hope to apply for an FDA review later this year. The United States is the only major country where silicone gel implants are limited, and women elsewhere overwhelmingly select silicone over saline when they have the choice. Reflecting its expectation that silicone implants are here to stay, Mentor just announced that it will build a large silicone implant facility in the Netherlands.

David Feigal of the FDA's Center for Devices and Radiological Health said the agency will surely hold a public advisory committee hearing on silicone implants.

"There are certainly remaining issues out there about silicone, but they shouldn't keep us from focusing on what we already know: that there is a likelihood that a woman with any implant will have to have additional surgery in the future," Feigal said. "Women need to know that once they go down the path of having breast implants, their bodies will be changed and there are predictable problems some of them will face."

Jennifer Moore is pleased with her breast implants, but some are worried about their effect on women's health.Lisa Marie Channing of Arlington says she had her breast implants removed after two months.