Doctors believe prostate cancer can be treated with female hormone replacement therapy without causing the side effects of normal care.
Prostate cancer treatment involves the suppression of testosterone
Hammersmith Hospitals NHS Trust and Imperial College London researchers had already shown HRT patches could help treat advance stage prostate cancer.
But the team have now claimed it helps to stop the weakening of the bones.
Prostate cancer treatment often involves suppression of testosterone, which can lead to osteoporosis.
Osteoporosis has traditionally been more common in women but it is on the rise in prostate cancer patients who survive for a long time.
Prostate cancer is the most common cancer in men, affecting 27,000 in the UK each year and killing 10,000.
The team found using HRT patches, commonly used to relieve menopausal symptoms in women, increased bone density by an average of more than 3% after a year on the 20 men treated.
The tumours were also regressing as giving the female hormone, oestrogen, shuts off testosterone production.
Testosterone can encourage cancer cell growth.
Lead researcher Paul Abel, a consultant urologist, said: "Patients on conventional prostate cancer treatment can lose up to 10% of bone mass as a side-effect in the first year of treatment alone.
"As long as treatment continues, so does bone loss, with a corresponding increasing risk of bone fracture with time."
He said the realisation of HRT treatment for cancer patients would be "good for patients".
"The considerable promise of oestrogen in advanced prostate cancer has already been reported by our team, but the additional benefit of preventing bone loss is extremely exciting news."
While use of HRT patches can cause breast growth, the team believe the side effect is outweighed by the benefits.
The team is now bidding for funding, with the backing of the Medical Research Council, for a larger study.
The Prostate Cancer Charity welcomed the research and said it hoped the team would win funding for a larger study.
Dr Chris Hiley, the charity's head of policy and research, said: "We know that osteoporosis is a common problem among men being treated for prostate cancer with hormone therapy.
"The team from Imperial College and Hammersmith Hospitals NHS Trust is conducting very interesting research and we hope it will be extended to a wider patient group in order to help improve the quality of life of men with prostate cancer."