Undergoing menopausal hormone therapy that combines estrogen with progestin for five years increases by 50 percent a woman's risk of breast cancer, according to a new study presented at a breast cancer conference in San Antonio.

“Women’s Health Initiative,” as the study was called, also showed that the recent drop in the incidence of breast cancer is mostly due to a drop in women having taken menopausal hormone therapy, said lead author Dr. Rowan Chlebowski of Harbor-UCLA Medical Center in Los Angeles.

In the study, researchers discovered that the risk of developing breast cancer increased when participants started taking the hormones, hit the highest point when the study ended and dropped after women had stopped taking the hormones.

In 2002, the study was halted after researchers noticed that thousands of women taking the estrogen and progestin pills had a higher risk of breast cancer and of heart problems. Since then, the use of hormones for postmenopausal women has declined by no less than 70 percent.

This is excellent news for women. They can still reduce the risk of breast cancer risk, by stopping the treatment, although they have been on hormones for some time, said Dr. Claudine Isaacs of Georgetown University's Lombardi Comprehensive Cancer Center. "It's not like smoking where you have to wait 10 or 15 years for the risk to come down."