Sometimes they lift. Sometimes they separate. But do underwire bras cause cancer? Could it be that the very garment designed to offer women support is actually killing them? That's the rumor that has been circulating for decades.

It all began in 1995 with a book called Dressed to Kill, in which Sydney Ross Singer and Soma Grismaijer, a husband and wife medical anthropologist team, claimed that women who wore tight-fitting bras all day, every day, had a much higher risk of developing breast cancer than those who went au naturel. The authors claimed that by inhibiting lymphatic drainage, bras trapped toxins in the breast tissue, which caused cancer.

According to critics, however, the bra-caused breast cancer theory is not supported by sufficient evidence. Scientists say the research of Singer and Grismaijer failed to exclude confounding variables such as the presence in some women of known risk factors for breast cancer. Thus, the notion of a correlation between wearing a bra and breast cancer does not appear to hold up.

"It just really is not logical in terms of what would increase your risk of breast cancer," says Louise Brinton, chief of the hormonal and reproductive epidemiology branch of the National Cancer Institute. Brinton, who has been doing research in the field for 30 years, says commonly accepted breast cancer risk factors are generally things that affect endogenous hormone levels.

These risk factors include how old a woman is and the age at which she had her first child. (The risk increases for women who have not had children, or who have given birth after the age of 30.) Breast-feeding and exercise are thought to lower risk, whereas a family history of the disease increases it. Scientists also know that 5 to 10 percent of breast cancers are linked to BRCA1 and BRCA2 gene mutations.

Marisa Weiss attributes some of the persistence of this urban legend to how frightening the reality of breast cancer can be. As president and founder of the Narberth, Pa.-based nonprofit Web site, Weiss, who has been in the field for two decades, sees women trying to figure out what in their everyday lives could cause the disease.

Weiss says that although the idea of having one's breasts in cages with metal wires "impeding fluid and marinating breast tissue in toxic liquid" sounds like a reasonable explanation for cancer, it is not. In fact, as she points out, far from being trapped, bodily fluid actually travels up and out of the armpits, not down toward the underwire.

Susan Love is president and medical director of the Dr. Susan Love Research Foundation in Pacific Palisades, Calif., and a former breast cancer surgeon as well as author of Dr. Susan Love's Breast Book, now in its fourth edition. Love agrees that the bra myth comes from the frustration of not knowing what causes the disease, coupled with a desire that the disease should come from the outside, from something a woman can control.

"You find people less wanting to think about birth control pills, hormone replacement therapy and fertility drugs," she says, "and more about pesticides, bras and deodorant. We don't know what causes breast cancer, and the majority of the risk factors that we know about do not explain it. However," she adds, "I don't think bras—or the lack thereof—are the secret answer."