A long-term study of men with low-risk prostate cancer has shown not actively treating the disease has a psychological impact.

Men diagnosed with low-risk prostate cancer who decide against surgery or radiation therapy have better sexual function but can suffer psychologically, a long-term study has found.

A decade ago there was a greater tendency to treat men with low-risk prostate cancer with surgery or radiation. Nowadays, the data suggests about 50 per cent are put on active surveillance, says Associate Professor David Smith, Senior Research Fellow at Cancer Council NSW.

While the vast majority of men with low-risk prostate cancer will live to a "ripe old age" without treatment, new research shows for a proportion of men just knowing they have cancer can have an impact on their mental wellbeing.

"Compared to patients who received an active treatment, such as prostatectomy or radiation therapy, those who remained on active surveillance or watchful waiting for up to ten years had greater fear of cancer recurrence, distress, hyperarousal and cognitive avoidance," Prof Smith said.

However these men did report fewer problems with urinary incontinence, and less sexual dysfunction, he said.

A study by Cancer Council NSW followed 340 men with low-risk prostate cancer for 10 years.

It found for the men who had treatment, 76 per cent experienced and reported sexual problems at 10 years. About 15 per cent reported urinary incontinence at 10 years.

The men who stayed on active surveillance - structured monitoring by a doctor - had significantly lower urinary incontinence and significantly better sexual function.

However these men reported greater rates of depression and anxiety as well as sleep problems.

"We measured nine different domains that relate to psychological issues and four out of those nine the men experienced worse psychological issues," Prof Smith said.

"Overall, our results suggest monitoring rather than actively treating men with low risk disease is better for long-term sexual health and reduced urinary leakage, but worse for long-term psychological outcomes."

The bottom line, he said, is that men need to be aware if they are having active surveillance that there are both benefits and costs.

"Active surveillance is a very appropriate treatment for many men with localised early stage prostate cancer but having a well-rounded picture about the positives and the negatives is important," Prof Smith told AAP.

If a man does choose against active treatment, then the findings of this study suggest its important to provide them with adequate psychological support, he said.

"Doctors also should be aware that when they are putting men on active surveillance that it's not just about treating the body, but there is also the potential to adequately counsel men to keep them safe and comfortable on that line of treatment," Prof Smith told AAP.

"There are all sorts of psychological tools and support for men who might be experiencing issues relating to anxiety around their prostate cancer."

Any man experiencing these issues is advised to speak with their doctor or to call the Cancer Council helpline on 13 11 20.