Studies are about to begin on 31,000 women at high risk for breast cancer to see whether a drug that blocks the sex hormone estrogen can protect them from the disease.

The medicine, tamoxifen, already is widely used to prevent recurrence of breast cancer in women who have been treated for the disease.Two studies, getting under way this year in the United States and Great Britain, will test whether the drug will protect women who are healthy but at high risk of breast cancer, the most common form of cancer in women.

About 44,000 women died of the disease last year, according to the American Cancer Society.

Many breast cancers require estrogen to grow. Tamoxifen appears to work by preventing estrogen from getting into the tumor cells and stimulating division.

Dr. Trevor Powles of Royal Marsden Hospital in London, director of the British study, said he expects the treatment will result in a 50 percent to 75 percent reduction in breast cancer.

In a pilot study on 1,500 women, Powles found the major side effect was hot flashes, which affected about 15 percent of the women.

The treatment had another potentially important benefit: The women's blood cholesterol levels fell by 15 percent. Powles said this may explain why deaths from heart disease decrease 40 percent among women who take tamoxifen for treatment of breast cancer.

The new British study will include 15,000 women, and the U.S. study, sponsored by the Food and Drug Administration, will involve 16,000 women.

These women will be at high risk of breast cancer because of such factors as having close relatives with breast cancer, having had benign breast lumps removed or having had no children, said Dr. Bernard Fisher of the University of Pittsburgh, head of the U.S. study.

Fisher and Powles discussed the research Tuesday at a meeting of the American Cancer Society.

In another report at the conference, Dr. Gianni Bonadonna of the University of Milan in Italy updated his study of breast-sparing surgery for breast cancer.

His work suggests that women may often be able to avoid total removal of their cancerous breasts if doctors first shrink their tumors by giving them chemotherapy and then remove just the cancerous lump.