One million Britons sign up for biggest study of women's health

The world's largest study of women's health has recruited more than one million Britons to settle the argument over the risks and benefits of hormone replacement therapy (HRT).

The world's largest study of women's health has recruited more than one million Britons to settle the argument over the risks and benefits of hormone replacement therapy (HRT).

Scientists will announce today that they have reached their target for the HRT study, which was launched three years ago. One in four of all women in the country aged 50 to 64 have signed up to join it. Almost 70,000 of the volunteers, who were recruited through the National Breast Screening Programme, are survivors of breast cancer.

Valerie Beral, head of the Imperial Cancer Research Fund's epidemiology unit at Oxford and principal co-ordinator of the study, said she was overwhelmed by the response. "There is very high use of HRT and anxiety about its effects. Women in this age group have concerns about their health as well as a willingness to contribute to medical research. We want to say thank you to all those who have volunteered," Professor Beral said.

"This is the biggest study ever done anywhere in the world. We will really be able to turn round and answer the questions people want answers to. Starting in a year's time we will be able to produce report after report."

The study, sponsored by the Imperial Cancer Research Fund, will assess the long-term risks of HRT, which has doubled in popularity since 1990. One in three women aged over 50 is taking HRT and one in two has tried it, but doctors are still unsure how they should advise patients about the health risks.

Research based on 51 studies from 21 countries, published in The Lancet medical journal in 1997 and also carried out by Professor Beral, concluded that there was a small increased risk of breast cancer in women who took the treatment. This increased risk amounted to two extra cancer cases occurring in every 1,000 women before the age of 70, on top of the 45 cases that would normally be expected. Specialists believe that this small extra risk is more than outweighed by a decrease in the risk of heart disease. It is also regarded as a valuable defence against osteoporosis, but may raise the risk of blood clots.

There are competing theories about the role of HRT in breast cancer. One possibility is that it delays the menopause and the fall in hormone levels associated with it, which would otherwise increase the incidence of the cancer. Another is that it may delay changes in the breast tissue at menopause which improve the effectiveness of screening.

Professor Beral said fears about the safety of HRT combined with the huge number of people taking it - estimated at two million - made the study essential.