Although these numbers are partially influenced by earlier cancer detection and longer life spans, no one knows why the incidence of breast cancer is growing. And breast cancer activists point out that mortality rates remain virtually the same as they were in the 1950s.

It also appears the disease is increasing in younger women. Roughly 30,000 women in their 40s will develop breast cancer this year, even though it is considered primarily a disease of aging. A leading cause of death for women in this age group, breast cancer tends to act more aggressively in younger women, doctors say.

Advocates for women with breast cancer also worry that mammograms may lull women into a false sense of security. Their recent efforts at lobbying Congress resulted in a pledge of $400 million for breast cancer research next year, almost three times what the government allocated in 1991.