Hundreds of older people in Mansfield in Ashfield may be living with undiagnosed dementia, according to estimates by the NHS.
Figures collected by GPs show that there are 1,049 people over 65 in Mansfield, and 1,181 in Ashfield who have been diagnosed with some form of dementia.
But estimates by the NHS, based on the age profile and gender of patients, suggest that the real figure may be 1,339 in Mansfield and 1,446 in Ashfield.
That means an estimated 555 pensioners in Mansfield and Ashfield all 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 the previous Prime Minster, David Cameron.
The Department of Health wants GP surgeries in England to increase the rate of diagnosis.
The latest data, for November, shows that the diagnosis rate for the whole of England is 68.2 per cent but area to area this ranges from 25 per cent to 90 per cent.
Both Mansfield and Ashfield are beating the national figure with a diagnosis rate of 78.4 per cent in Mansfield and 81.7 per cent in Ashfield.
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 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 as people with dementia deserve better.”
The Royal College of General Practitioners (RCGP) said that doctors realised the importance of spotting the signs of dementia but in some circumstances an early formal diagnosis was not in the best interests of the patient.
Prof Helen Stokes-Lampard, chairman of the RCGP, said: “We recognise that timely diagnosis of dementia can be helpful in that it provides an opportunity for some patients to get their affairs in order and make long-term plans for their care.
“But 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."
Dementia is a term used to describe symptoms such as loss of memory, behaviour changes and problems in reasoning.
The most common cause of dementia is Alzheimer's disease, which accounts for about 60 per cent of cases, but it can be the result of brain damage caused by a stroke or neurological conditions such as Parkinson's.
Dementia can affect people at any age and across England 465,000 people have been diagnosed.
But the vast majority of cases are in older people.