Breast implants may cause a small but significant increase in the risk of an extremely rare but treatable type of cancer, the Food and Drug Administration said on Wednesday.

The risk applies to both saline- and silicone-filled implants, and to all women who have them, whether for reconstruction after cancer surgery or for cosmetic enlargement of the breasts.

The cancer, anaplastic large-cell lymphoma, involves the immune system. It is not breast cancer. It is usually a systemic disease, but in the cases linked to implants, the lymphoma grew in the breast, usually in the capsule of scar tissue that formed around the implant. The cases were discovered because women developed symptoms long after they had healed from the implant surgery - lumps, pain, asymmetry of the breasts, fluid buildup and swelling.

In some cases simply removing the implant and scar tissue gets rid of the disease, but some women might need chemotherapy and radiation, said Dr. William Maisel, the chief scientist and deputy director for science in the drug agency's Center for Devices and Radiological Health. He said there was some evidence, though not conclusive, that the form of this lymphoma found in implant patients was less aggressive than the usual type.

So far, the drug agency said it knew of about 60 cases worldwide, a tiny number compared with the 5 million to 10 million women who have implants. But even that small number appears to be an excess of cases when compared with the usual incidence of the disease: This type of lymphoma in the breast is normally found in only 3 in 100 million women who do not have implants.

Because the risk appears to be so small, the drug agency said, "the existing data support the continued marketing and use of breast implants." But it also said that women considering implants should first discuss the information with their doctors.

Women with implants who have no symptoms do not need to do anything special or change their routine health care, Dr. Maisel said. But they should pay attention to any changes and see a doctor if swelling, lumps or other symptoms develop.

Because there is very little data on the lymphoma, Dr. Maisel said, the agency "is not comfortable recommending a specific treatment." Cases may differ, and women who have the disease need to be treated by a breast surgeon and an oncologist.

The worldwide market for breast implants is roughly $820 million a year and growing at 8 percent a year, according to industry figures.

The Food and Drug Administration said it needed more data and was asking doctors to report confirmed cases to its safety information program, MedWatch. Dr. Maisel said the agency was working with the American Society of Plastic Surgeons to create a registry of cases. He also said that information about the possible link to the cancer would be added to the labeling information sent out with implants.

Dr. Maisel said it was not yet known for sure whether the implants really increase the risk of the cancer, or how they could do so.

"We do know that both silicone and saline implants are surrounded by silicone," he said, adding that silicone has been found in cells around breast implants and may stimulate the cells and induce lymphoma in rare cases. But he added, "Please understand that is speculative and a hypothesis."

A spokeswoman for Allergan, which makes implants, sent a statement by e-mail saying that the company supported the drug agency's recommendations. But the statement also said: "Reports of ALCL in patients with breast implants are extremely rare and are not to be mistaken for breast cancer. A woman is more likely to be struck by lightning than get this condition."

Another implant maker, Mentor, did not respond to a telephone message asking for comment on the drug agency announcement.

A medical group, the American Society for Aesthetic Plastic Surgery, issued a statement saying it supported the development of a registry for the cancer cases in women with implants, but added that implants were "the most studied device in medical history" and were safe.