A new genetic test has been developed that accurately distinguishes between "pussycat" and "tiger" forms of prostate cancer.
By providing a reliable way to tell apart slow-growing and aggressive cancer, the biopsy test can help doctors provide better treatment options.
Men with mild "pussycat" prostate cancers may not experience symptoms or need treatment throughout the rest of their lifetime.
Others, with aggressive "tigers", risk going to an early grave unless immediate action is taken.
One of the biggest problems involved in treating prostate cancer is knowing in advance what kind of disease a patient has.
The PSA blood test used in prostate cancer diagnosis is notoriously unreliable, often providing misleading results.
Biopsy samples provide a one to 10 "Gleason score" of aggressiveness, but even this is not a foolproof guide.
The new Prolaris test, which has been evaluated by an international team of researchers, measures the activity of genes that drive cell division and provides a Cell Cycle Progression (CCP) score.
A one unit increase in CCP score was found roughly to double the risk of prostate cancer death or recurrence.
Professor Jack Cuzick, a study author based at Queen Mary, University of London, said: "Over-treatment of prostate cancer is a serious issue so it's essential that we have an accurate way of spotting those cancers that pose an immediate risk.
"For patients with slow-growing tumours, it's far safer and kinder to watch and wait - only acting if the situation starts to change.
"We've shown this test is accurate at telling apart these two different tumour types at many different stages of treatment. But we still need to work out how best to use this test to help patients.
"We want to try and shorten the time it takes to get the results and establish how frequently the test needs to be done in order to be most effective at spotting any changes."
Dr Harpal Kumar, chief executive of Cancer Research UK, and chair of the National Cancer Research Institute, said: "As we've learnt from breast cancer, you often have to balance the potential harms and benefits of screening.
"Some countries use PSA testing, which uses a blood test to look for increased levels of a hormone associated with prostate cancer. But this doesn't tell you whether the tumour is aggressive or not.
"Being able to tell apart aggressive and slow-growing tumours would help us take a major step forward in prostate cancer treatment.
"Understanding more about the nature of a patient's tumour could spare thousands of men from unnecessary treatment and the resulting side effects, whilst also meaning that those who do need treatment receive it rapidly."
Dr Iain Frame, director of research at Prostate Cancer UK, said: "Developing an effective test to distinguish aggressive from non-aggressive prostate cancer could be a game changer for those affected by the condition.
"As it stands the current diagnostic process is at best inadequate and means some men's cancers are left untreated until it is too late, while other men are left with life-changing side effects from treatment for a condition which might never have caused harm."
Each year prostate cancer affects around 41,000 men in the UK and causes 11,000 deaths.