The largest study ever conducted on women who take estrogen during menopause concludes the medicine can cause cancer of the uterus and that the risk increases the longer it is used.

Estrogen should be prescribed "only for important indications when the benefits seem to outweigh the risk," it said.

The study, conducted at Johns Hopkins University and published in yesterday's edition of the New England Journal of Medicine, contradicts a Yale University report published two months ago which concluded that the link between estrongen and cancer is based on faulty research.

Estrogen is a natural female hormone that is widely prescribed to ease the discomforts of menopause in middle-aged women.

"The new work should dispel controversy about the association between estrogen use and uterine cancer," Food and Drug Commissioner Donald Kennedy said in Washington. "Women now taking or considering taking these drugs should read carefully the information provided with them and discuss the drugs will their doctors."

The study examined 1.339 women in menopause and concluded that those who take estrogen pills are six times more likely than nonusers to have cancer of the uterine lining. For those who use the medicine for more than five years, the risk is 15 times greater.