Hormone replacement therapy increases the risk of ovarian cancer, analysis of 52 separate studies has found.
The University of Oxford research, in the Lancet, found an extra case for every 1,000 women taking the drugs for five years from the age of 50.
Lead researcher Sir Richard Peto said claims there was no risk for short courses of HRT "simply isn't true".
A leading cancer charity said this was a "modest increase on a relatively uncommon cancer."
Medical charities said the findings were robust, but said the risk did fall after HRT stopped.
HRT drugs are used to alleviate the symptoms of the menopause, which can be so severe they interfere with day-to-day life.
In the UK, women take HRT mostly for between two and five years.
There is an increased risk of breast cancer, which has been well documented. However, the drugs have also been found to reduce the risk of osteoporosis and bowel cancers.
But the impact on ovarian cancer has been debated, particularly for those taking the drug for less than five years.
Scientists behind the latest study, which looked at data from 52 separate studies including 21,500 women with ovarian cancer, say it proves there is a link.
Prof Sir Richard Peto told the BBC News website: "It's a risk, about a million women in this country have HRT and 1,000 will get ovarian cancer from it."
He specified there would be one extra death for every 1,700 people taking hormone replacement therapy.
He said the risks would be reduced for those taking it for a shorter period of time but were still there.
He added: "If it were me and I had really bad symptoms, I'd worry more about those than any possible risk."
Theresa Chaffey, from Bournemouth, told the BBC News website: "I wasn't actually given any information in regards to any breast cancer risk or ovarian cancer risk.
"Hearing the news today is very worrying because of actually being on HRT for so long and with GPs continually saying I need to stay on it for my bones as well as any symptoms."
'Unbearable' without HRT
But Val Weedon from Chatham in Kent commented: "I have been taking HRT in one form or another since 1997 when I had a full hysterectomy.
"My life would be unbearable if I don't have HRT and for a number of other women who suffer severe menopausal symptoms it is the same. We have to weigh up the quality of life alongside the risks, and therefore HRT is a risk we'd rather take than have no life."
Ovarian cancer is often difficult to diagnose as the symptoms are similar to irritable bowel syndrome and other conditions.
The main symptoms include:
- increased abdominal size and persistent bloating (not bloating that comes and goes)
- persistent pelvic and abdominal pain
- difficulty eating and feeling full quickly, or feeling nauseous
Commenting on the findings, Prof Montserrat Garcia-Closas, from the Institute of Cancer Research in London, told the BBC: "There's been a question for a long time about how solid the evidence was and this paper shows really very convincingly that there is an association."
But she cautioned: "It's a modest increase on a relatively uncommon cancer.
"For the general impact on a woman of average risk, then breast cancer will be a more important consideration than ovarian cancer."
Katherine Taylor, from the charity Ovarian Cancer Action, said: "Every woman is different though, and may each have different risk factors."
She said some women would be at increased risk because of their family history and the inheritance of some cancer risk genes, such as BRCA.
"Knowing her BRCA status could affect a woman's decisions around her health care. It could help her to decide whether HRT is the right path, for example."
The UK's medicine's regulator said it advised using the lowest does for the shortest possible time.