The finding comes from Valerie Beral, MD, and colleagues with the U.K. Million Women Study.
They invited more than a million women living in Britain to take part in the study, starting in 1996.
Beral's team found that over a five-year period, women currently using hormone replacement therapy (HRT) for at least five years were 20% more likely to develop ovarian cancer and 23% more likely to die of ovarian cancer than women who had never used HRT.
Based on those findings, the researchers write that "since 1991, use of HRT has resulted in some 1,300 additional ovarian cancers and 1,000 additional deaths from the malignancy in the U.K."
However, the American Society for Reproductive Medicine notes that short-term HRT use probably isn't harmful for women using HRT to ease menopause symptoms.
When the study started, the women were about 57 years old, on average. They reported their current and past HRT use. Nearly two-thirds completed a follow-up HRT survey about three years later.
Half of the women had ever used HRT. They include 30% who currently used HRT and 20% who had used HRT in the past.
The researchers followed the women for five years, on average. During that time, 2,273 women were diagnosed with ovarian cancer and 1,591 of those women died of ovarian cancer.
HRT Study's Results
Over five years, there was one extra ovarian cancer in roughly 2,500 HRT users and one extra ovarian cancer death in about 3,300 HRT users, Beral's team notes.
The increased risk of ovarian cancer only occurred in women who currently used HRT and had done so for at least five years.
Women who had quit HRT use -- and those who had used HRT for less than five years -- weren't at increased risk of ovarian cancer, compared to women who had never used HRT.
The type of HRT didn't affect the results.
The study, published in The Lancet, doesn't prove that HRT causes ovarian cancers.
But the results held when the researchers adjusted for factors including age, time since menopause, smoking, physical activity, age at first birth, BMI (body mass index), alcohol consumption, past use of oral contraceptives, social class, and geographic location.
The ovarian cancer risk seen in the study "might be thought of as small, but enormous numbers of women have been exposed" through HRT use, states an editorial in The Lancet.
HRT use has dropped based on previous studies showing other health risks with long-term HRT use, notes editorialist Steven Narod, MD, FRCP.
Narod directs the Familial Breast Cancer Research Unit at the Women's College Research Institute and is also a professor in the public health sciences department at the University of Toronto.
"With these new data on ovarian cancer, we expect the use of HRT to fall further," writes Narod. We hope that the number of women dying of ovarian cancer will decline as well."
Short-Term HRT Use
The American Society for Reproductive Medicine (ASRM) issued a statement about the study.
"Although long-term use of estrogen appears to increase the risk of ovarian cancer, women should be reassured that short-term use for symptomatic treatment at or near menopause is unlikely to increase the risk of ovarian cancer appreciably," says Robert Rebar, MD, ASRM executive director, in the statement.
"These data, added together with the most recent data from the Women's Health Initiative, provide reassurance all-in-all that short-term use of estrogen is not harmful to symptomatic women," says Rebar.