Men who ejaculate more regularly throughout their lives lower their risk of the disease.
The researchers, from Harvard Medical School, did not explain why orgasms could lower prostate cancer risk.
However it has previously been theorised that regular orgasms may flush out cancer-causing chemicals in the prostate.
Another theory is that if sperm is regularly cleaned out to allow new cells to develop, it helps stop the build-up of old cells that might be more likely to turn cancerous.
The prostate is a small satsuma-sized gland located between a man's penis and his bladder, whose main function is to produce a thick white fluid that is mixed with the sperm produced by the testicles, to create semen.
The new study is the largest to date on the frequency of ejaculation and and prostate cancer.
The researchers found that men in the 40-49 age bracket who ejaculate 21 or more times a month reduced their risk of prostate cancer by 22 per cent.
This was compared to men who ejaculate four to seven times a month.
To protect against prostate cancer, take a lover - or 20.
According to a study, men who sleep with multiple women are almost a third less likely to develop the disease.
Researchers at the University of Montreal found men who have more than 20 notches on their bedpost slashed their risk of prostate cancer by 28 per cent.
And the study also revealed that men who have slept with more than 20 women reduced their chances of getting the most aggressive tumours by 19 per cent.
Celibacy, on the other hand, doubles the risk of the disease.
The research was published in the journal Cancer Epidemiology.
While the researchers said they were unclear as to why ejaculation lowers the chances of prostate cancer, they called the results ‘particularly encouraging.’
The study followed almost 32,000 healthy men for 18 years, 3,839 of whom later were diagnosed with prostate cancer.
Men were asked about their average monthly frequency of ejaculation between the ages of 20 to 29, 40 to 49, and in 1991, the year prior to the questionnaire.
They found that the more frequently a man ejaculated throughout his life, the lower his risk of prostate cancer at all three of these points in time.
This was the case even when they adjusted their results to take factors such as diet, lifestyle and a history of prostate cancer screening into account.
Dr Jennifer Rider, of Harvard Medical School and Brigham and Women’s Hospital, said the results are ‘particularly encouraging’ but should be interpreted with caution.
She said: ‘While these data are the most compelling to date on the potential benefit of ejaculation on prostate cancer development, they are observational data and should be interpreted somewhat cautiously.
‘At the same time, given the lack of modifiable risk factors for prostate cancer, the results of this study are particularly encouraging.’
She added more research should be carried out into the specific changes in the prostate caused by ejaculation, to understand how it reduces the risk of prostate cancer.
The research was presented at the American Urological Society annual meeting in New Orleans in May.
More than 1.1 million cases of prostate cancer were recorded in 2012, accounting for around 8 per cent of all new cancer cases and 15 per cent in men, according to figures from the World Cancer Research Fund International.
The causes of prostate cancer are largely unknown, but the chances of developing the disease increase as a man gets older.
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