Overdiagnosis
Correctly diagnosing something real that would never have caused you any harm.
What it means
Finding a condition that is genuinely there, but which would never have caused symptoms or shortened your life.
It is not a false positive. The diagnosis is correct. The problem is that it did not need making — and once made, it usually gets treated.
It happens because some cancers grow so slowly, or not at all, that something else would have got there first — and there is currently no reliable way to tell those apart from the dangerous ones.
It is the main cost of cancer screening, and it is why the numbers on our screening pages give both the deaths prevented and the people overdiagnosed. In breast screening, roughly three women are overdiagnosed for every death prevented.
In practice
In NHS screening leaflets, which do cover it — and which are worth reading properly rather than skimming.
In debates about prostate, thyroid and breast screening in particular, where it is the central issue.
Why it matters
Read the numbers before deciding about screening. Both figures — benefit and overdiagnosis — are published, and you are entitled to weigh them yourself.
Understand that it is invisible at the individual level. Nobody can tell you whether your particular diagnosis was an overdiagnosis, which is precisely what makes it difficult.
It is a genuine argument for thinking before testing, and not an argument against screening as a whole — for bowel and cervical screening, the balance is clearly favourable.
Clinically reviewed by Dr Mohammad Zubair Khan, GMC 7563469
Last reviewed
August 30, 2026