A study by Cancer Council NSW has found that men with more sexual partners have an increased risk of developing prostate cancer.

The more sexual partners a man's had the greater his risk of developing prostate cancer.

A study by Cancer Council NSW published in the International Journal of Cancer has found sexual activity is among the risk factors that at least doubles the risk of the disease.

"Our study points towards men who engage in higher levels of sexual activity and start being sexually active earlier as being at a higher risk of prostate cancer," said Dr Visalini Nair-Shalliker, a research fellow at Cancer Council NSW.

The following factors at least double the risk of prostate cancer in men: having a father with a history of prostate cancer, a previous diagnosis of prostatitis or benign prostatic hyperplasia, the number of sexual partners in a lifetime, with the risk being highest in the group that reported the most sexual partners.

Previous research had pointed to greater sexual activity in early adulthood as reducing later risk.

"The role of sexual activity is just not straightforward, due to its multifaceted nature. It's an area that needs further exploration," Dr Nair-Shalliker added.

It's believed this increased risk associated with sexual activity could be due to hormonal changes.

"More research into the exact mechanisms is needed, but what we do know is that a number of risk factors we looked at - for example, obesity and sexual activity - are associated with hormonal activity. Hormonal changes have been linked to prostate cancer initiation, so that's the common potential underlying cause that we can see at this stage," said Dr Nair-Shalliker.

Being overweight or obese was associated with increased risk, but to a lesser extent.

"The impact of obesity on the risk of developing prostate cancer is a concern, with 70 per cent of Australian men being overweight or obese," Dr Nair-Shalliker concluded.

There was no association found between prostate cancer risk and circumcision, vasectomy or erectile function.