WASHINGTON Jan 7 - Women who smoke may have a far higher risk of breast cancer than those who do not, or those who once smoked but quit, U.S. researchers reported on Tuesday.

California women who said they were current smokers had a 30 percent greater incidence of breast cancer than non-smokers, the researchers reported in this week's issue of the Journal of the National Cancer Institute. Peggy Reynolds and colleagues at the California Department of Health Services studied 16,544 women between 1996 and 2000.

During that time, 2,005 of them were diagnosed with invasive breast cancer.

Women who described themselves as current smokers had a 30 percent higher risk of being among the cancer patients. Those who started smoking before age 20, who began smoking at least five years before their first full-term pregnancy, and who smoked the most or the longest all had higher risks.

Women who had once smoked but quit did not have a higher risk of breast cancer, Reynolds' team found. Passive smoking also did not seem to raise breast cancer risk in the California study.

Breast cancer is the third most common cause of cancer death in the United States, after lung cancer and colon cancer. It killed 40,000 women in 2003, according to the American Cancer Society.

The researchers said their study helps shed light on an area where studies have had conflicting results. They now plan to run genetic tests on the women in the study to see if a genetic mutation may make certain women more susceptible to the cancer-causing effects of tobacco smoke.

Tobacco smoke carries several known carcinogens, and elements of tobacco smoke have been found in the breast fluid of smokers, they noted. But tobacco could also affect estrogen in ways that, theoretically at least, could lower breast cancer risk. The study tends to refute that notion. - Reuters