Fewer than half of people eligible for a NHS health check in England have taken up the offer, despite it being free to everyone over 40.
The routine check can pick up heart problems early and help to prevent dementia, plus highlight the need to stop smoking, NHS England said.
The check-up takes 20 minutes and is carried out by a GP or nurse.
It involves tests on blood pressure, weight and height, and is offered every five years up to the age of 74.
Around 7.15 million people have had a free health check since 2013, yet 15 million are eligible, NHS stats show.
NHS England said identifying people with an irregular heartbeat or high blood pressure and giving them treatment would help to prevent dangerous blood clots which could lead to strokes.
And if more people went for their health check, the number of people having a stroke or other heart problems would reduce, potentially preventing thousands of cases of vascular dementia.
Who can have an NHS health check?
Anyone between the age of 40 and 74 who is generally healthy and doesn't already have any diseases.
You should automatically get an invitation through your GP surgery or local authority every five years.
After the age of 74 you can request a health check-up from your GP or nurse.
What problems can they spot?
By checking blood pressure, family history, lifestyle, height and weight, it is possible to give an idea of your risk of getting heart disease, stroke, kidney disease and type 2 diabetes.
Research shows that people with a history of heart diseases are at least twice as likely to develop vascular dementia.
You will receive personalised advice on how to lower your risk of all those conditions, usually by:
- improving your diet
- taking more exercise
- taking medicines to lower blood pressure or cholesterol
- losing weight
- stopping smoking
If you are 65 or over, you will be told the signs and symptoms of dementia to look out for.
Why this could be a 'life-saving step'
Some people told the BBC the check had been time consuming.
Anna Oxbury, from Cheltenham, said she had to go to her local hospital for the blood testing part of the check because her GP practice was unable to do it.
She said: "I'm self-employed and reckon the 'free' health check, which meant three separate visits to practice and hospital, cost me over £100 in lost work time.
"My husband has an important job and could only make time for the check if he took a half-day holiday. That's why he won't do it."
However, David, from Hampshire, said the check "probably saved his life".
A letter for the appointment went out to him in October after he turned 50.
The blood test revealed he had developed diabetes - thought to be the type-2 version of the condition - and his GP was concerned enough about the results to send him to the hospital for further tests.
There they found he was severely dehydrated and had to give him three and a half litres of saline to rehydrate him.
"That routine test probably saved my life," he said.
Alistair Burns, national clinical director for dementia and older people's mental health for NHS England, said: "Heart disease and dementia are two of the biggest health risks facing people in our country.
"Attending a free NHS health check is a great opportunity to discuss existing health conditions, and to work out how to reduce the risk of developing dementia and other illness in the future."
He said the start of a new year was the right time to commit to "taking a simple, free and potentially life-saving step towards a healthier life".
Public Health England said the check looked at the top causes of premature death and ill health and supported people to take action to reduce their risks.