Nearly 4,000 people over the age of 65 in Norfolk are living with undiagnosed dementia, according to the NHS.
Figures collected by GPs show there are 6,208 people living with some form of dementia in the county.
But estimates by the health service, based on the age profile and gender of patients, suggest that the real figure may be 10,037.
It means an estimated 3,829 pensioners living with a debilitating illness that has not been formally recorded by their doctor.
The figures are being collected in response to the prime minister's challenge on dementia set up by David Cameron.
The Department of Health wants GP surgeries in England to increase the rate of diagnosis.
While Great Yarmouth and Norwich both exceeded the diagnosis rate for England, which is 68.2pc, other areas of Norfolk did worse.
South Norfolk had a rate of 57.5pc, North Norfolk had 58pc, while King's Lynn and West Norfolk had 59.3pc.
Sally Copley, Alzheimer's Society's director of policy campaigns and partnerships said: 'The essential first step to the care and support everyone should have a right to is a diagnosis.
'Research has found that people with dementia who live alone, or in care homes, or from black, Asian, and minority ethnic (BAME) communities, have lower rates of diagnosis – contributing to the variation we're seeing across the country.
'People tell us that our support is a lifeline, but we cannot reach them if we do not know who they are.
'In areas with low diagnosis rates the government has to work with the NHS to find out what's going on. People with dementia deserve better.'
However Professor Helen Stokes-Lampard, chair of the Royal College of GPs, said: 'Pressuring patients to seek early advice and a diagnostic label, especially for short-time memory lapses, can create considerable worry for both them and their loved ones, especially when follow-on services and treatment options are limited.
'There may also be some instances where GPs might decide to delay giving a formal diagnostic label, especially in the early stages, if there is minimal impact on the quality of a patient's day-to-day life.'
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules here