Many men diagnosed with low-grade prostate cancer do not benefit from radical treatment, research suggests.
Some patients are given radiotherapy
The researchers calculated that, even without treatment, only about 1% of men aged 55-59 with diagnosed low-grade disease would die within 15 years.
Side effects of radical treatment such as surgery and radiotherapy can include incontinence and impotence.
The Department of Health said its advisers would consider the Institute of Cancer Research findings.
The study appears in the British Journal of Cancer.
Prostate cancer is the most commonly diagnosed male cancer in the UK.
Nearly 32,000 new cases are diagnosed, and around 10,000 men die from the disease, each year.
At present, men diagnosed with the disease may undergo radical treatment - either surgery to remove the prostate or radiotherapy.
Alternatively, they may simply be managed by observation - a technique known as watchful waiting.
The Institute of Cancer Research team found that radical treatment was only effective for men with high-grade disease.
In those cases they calculated that, without treatment, up to 68% could die from prostate cancer.
Researcher Dr Chris Parker said: "Most men with prostate cancer detected by PSA screening will live out their natural span without the disease ever causing them any ill effects.
"The decision whether to have radical treatment can be tremendously difficult for the patient.
"The results of trials looking at the long-term survival benefit of radical treatment are several years away.
"So, this new information on the potential impact of treatment on overall survival will be of great interest to men faced with this decision."
Dr Parker said his team was trialling a new prostate cancer management technique called active surveillance.
This aims to target treatment only at those who need it by closely monitoring patients for signs of disease progression.
Preliminary results of this technique have been encouraging.
Types of cell
High-grade prostate cancers are made up of undifferentiated cells, which can reproduce quickly, speeding growth of the tumour.
Low-grade tumours are made up of differentiated cells which do not reproduce at the same speed.
Chris Hiley, from the Prostate Cancer Charity, said: "Decision making on treatment for prostate cancer is not straightforward for anyone involved, but we hope that these results might make explaining options and possible outcomes to patients easier for doctors.
"Clearly, some men with a prostate cancer diagnosis will always prefer an operation to cut it out or radiotherapy to treat the cancer.
"This new evidence shows men mustn't be left to overestimate the survival advantage that such an option would give them."
Dr Emma Knight, of Cancer Research UK, said: "It is important to stress that these results are only predictions.
"Data from ongoing clinical trials should, in time, portray the pros and cons of treatment versus monitoring more accurately."
The Department of Health said the findings would be considered by its Prostate Cancer Advisory Group.