A large federal study of estrogen therapy in postmenopausal women has been stopped a year ahead of schedule because the hormone increased the risk of stroke and offered no protection against heart disease, the government announced Tuesday.

The women in the study, aged 50 to 79, took estrogen alone, with no other hormones, for an average of seven years.

The National Institutes of Health, which sponsored the study, issued a statement saying, "The NIH believes that an increased risk of stroke is not acceptable in healthy women in a research study."

But the researchers also noted that the increased risk is small, estimated at about 8 extra strokes per year for every 10,000 women taking estrogen.

The study also found that estrogen decreased the risk of hip fractures and had no effect on breast cancer risk. A related study suggested that estrogen may increase the risk of dementia and mild cognitive impairment.

"Women should not feel that this is some grand emergency for them," said Dr. Barbara Alving, director of the Women's Health Initiative, a large project of which the estrogen study was a part. She said that although women in the study have been instructed to stop taking their pills, other women who use estrogen should discuss the findings with their doctors to decide whether to continue.

About 4 million women in the United States take Premarin, the form of estrogen used in the study.

The Food and Drug Administration said Tuesday that it would assess the findings to determine whether estrogen labels and directions to patients should be changed to reflect the new information.

Another study in the Women's Health Initiative, in which women took two hormones, estrogen and progestin, was also stopped early, in July 2002, because the combined hormones increased the risk of breast cancer. But in the study now being halted, estrogen alone did not increase the risk of breast cancer.

The study that was stopped on Tuesday involved 11,000 women who were assigned at random to take either estrogen tablets or a placebo and were followed for an average of seven years. Only women without a uterus can safely take estrogen alone, because by itself the hormone can increase the risk of uterine cancer. Therefore, all the women in the study had had hysterectomies. The study was supposed to continue until 2005, but letters have been sent to the women instructing them to stop taking the pills immediately.

Drug makers and consumer groups disagreed on the impact of the new findings.

Cindy Pearson, executive director of the National Women's Health Network, a consumer group, said, "What is important is that all those hopes of the last 30 years, that in one version of a hormone pill or another there existed a fountain of youth, those hopes are dashed."

She added: "I was most struck by the trend toward a possible increase in dementia. To see a risk, apparently from estrogen alone, that's really shocking."

But the drug company Berlex Laboratories, which makes estrogen products and is based in Montville, N.J., issued a statement that highlighted the bone-protecting effects of estrogen, the fact that it did not increase the risks of breast cancer or heart disease in the new study, and its proven ability to ease hot flashes and the vaginal discomfort sometimes caused by menopause.