Almost 75% of Britons are unaware of the health indicators that could save their lives, a survey suggests.
Nearly half of those interviewed were aware of their blood pressure
Some 84% consider themselves healthy but many are said to have no knowledge of cholesterol, blood pressure, body mass index or family illness history.
Doctors say it is important to be aware of the "vital stats" as they are health monitors and can help to predict the risk of coronary heart disease.
TNS spoke to 1,027 people in March for health insurer Pru Health.
This survey comes after the government launched an initiative to encourage people to make small changes to improve their health such as stopping smoking, doing more exercise and eating better.
Pru Health said if its findings were extrapolated out to cover the whole population, some 33 million British adults could be said to be "health illiterate" with little or no knowledge of the key data.
Some 38% of those interviewed said they were overweight but only 25% of people knew their BMI, and 60% were unaware it related to their weight to height ratio.
BMI is calculated by dividing weight in kilograms by the square of your height in metres.
A BMI of 25 to 29.9 is considered overweight and one of 30 or above is considered obese.
Nearly half knew their blood pressure and while cholesterol levels were known by 29% of respondents, this dropped to less than 10% among the under-35s.
Reducing blood pressure has been shown to reduce the risk of having a heart attack.
Pru Health it was crucial people know their cholesterol levels because currently nearly half of all people who die from coronary heart disease in Britain have a raised cholesterol level.
Only 12% of people said they were aware of illnesses of parents, grandparents and other relatives.
"Having a vague understanding of your height and weight, and thinking you need to lose a few pounds is simply not enough to keep healthy," said Pru Health's medical director Dr Chris Dark.
"Instead you need to know your Vital Stats and review them regularly. It's worth it because in the long run, it could help reduce and prevent future problems and, ultimately, save your life."
Belinda Linden, of the British Heart Foundation, said the survey's findings were a concern.
"When we know the facts, we don't seem to be able to act on the lifestyle messages," she said.
"While many people are aware of their need to eat a healthy diet, to lose weight and to be more active, we are still finding a rising tide of obesity and inactivity across the country which can cut our lives short through ill health."