The 24-year, phase III international trial provides the strongest evidence yet that women with epithelial ovarian cancer - which accounts for 80-90 per cent of cases - can safely take HRT during or after their treatment.
Several major studies have found that HRT can increase the risk of developing some cancers, which is why there has been such interest in whether it is safe to take during cancer treatment.
The new study, published in the Journal of Clinical Oncology, was led by researchers at The Institute of Cancer Research, London, and The Royal Marsden NHS Foundation Trust, and is the largest to investigate HRT's effects on ovarian cancer survival.
It was funded by The Institute of Cancer Research and Cancer Research UK, and endorsed by the UK Co-ordinating Committee for Cancer Research.
Set up in 1990, the trial followed 150 women with epithelial ovarian cancer - half of whom were allocated to receive HRT during their treatment, and half of whom were not - and compared overall survival at a point in 2012, 22 years after the trial began.
Women were recruited from 19 different hospitals across the UK, Spain and Hungary, in a study co-ordinated by researchers at The Institute of Cancer Research (ICR).
53 of the 75 women (71 per cent) in the HRT arm of the trial - who were allocated to receive HRT for up to five years after the study began - had died, compared with 68 (91 per cent) of the 75 women who had not taken HRT.
The results are relevant to a large proportion of women with ovarian cancer because ovarian cancer treatments can trigger the menopause.
The study suggests that women with ovarian cancer can receive the known benefits of HRT on the side-effects of the menopause, without it reducing their survival chances. In fact, the results indicate that HRT might improve chances for overall survival.
The trial looked at women who already had ovarian cancer. Years of research have already shown that when healthy women take HRT their risk of developing breast, ovarian and possibly womb cancer can increase.
Study clinical lead Professor Ros Eeles, Professor of Oncogenetics at The Institute of Cancer Research, London, and Consultant at The Royal Marsden NHS Foundation Trust, said:
"We were really happy to be able to show that HRT is safe for women with the most common type of ovarian cancer. Whether or not to have HRT is a very important decision for a large proportion of women with ovarian cancer, who will often have to undergo the menopause due to the cancer treatment at the same time as coping with a cancer diagnosis.
"Our results not only suggest HRT is safe for women with this type of ovarian cancer, but that it may actually improve their chances of long-term survival. We hope our study will inform treatment for women with ovarian cancer, and the findings could have a big impact on their quality of life."
Study scientific lead Professor Judith Bliss, Director of the Cancer Research UK-funded Clinical Trials & Statistics Unit at The Institute of Cancer Research, London, said:
"Our results should provide some reassurance to women with ovarian cancer who until now might have avoided HRT because of worries about its effect on their cancer. HRT does not seem to have a detrimental effect on survival in women with epithelial ovarian cancer, and there is even the intriguing suggestion that it may be improving their survival chances. We would like to see more research into this area in the future and hope to see other studies confirm this finding in larger numbers of women."
Fiona Osgun, health information officer at Cancer Research UK, said:
"Years of research has shown that women taking HRT are at an increased risk of breast and ovarian cancer, as well as endometrial cancer for some forms of HRT. This research takes a different approach and looks at women already diagnosed with ovarian cancer.
"These results are a great first step in helping to understand if HRT is safe to take for women with ovarian cancer, but we need larger studies with more women to confirm them. HRT has been shown to be effective at reducing symptoms of the menopause and there are many factors that play into a woman's decision to use it or not. If you are thinking of stopping or starting HRT speak to your GP."