Carrying extra fat around your middle dramatically increases your risk of early death, even if your overall weight is normal, say researchers.
A study of almost 360,000 people from nine European countries found waist size a "powerful indicator" of risk.
Each extra 2ins (5cm) raised the chance of early death by between 13% and 17%.
The New England Journal of Medicine study stressed GPs should regularly measure patients' waists as a cheap and easy way to assess health.
It costs virtually nothing to measure your hip and waist size
Professor Elio Riboli Imperial College London
The link between waist fat and health problems has been established for some time, but the sheer size of the study gives scientists a far more accurate picture.
The researchers, including some from Imperial College London, followed the volunteers, who were an average of 51 years old at the start of the study, for the next 10 years, during which time 14,723 of them died.
The standard measure of obesity, body mass index (BMI) remained a reasonable predictor of health problems, with those with a high reading more likely to die from cardiovascular disease or cancer.
However, the 'hip/waist ratio', a number produced by dividing the waist size by the hip measurement, and just the waist measurement on its own, were both good ways of sorting out those at highest risk.
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Some people who had a completely normal BMI score, but a larger than average waist, were at significantly higher risk of early death.
At the extremes, men with waists exceeding 47ins (119cm) had a doubled rate of death compared with those with waists under 31.5ins (80cm), and a similar statistic was found when women with waists over 39ins (99cm) were compared to those under 25.5ins (64.7cm).
An increase in risk of death could be plotted every time the belt was let out by another two inches - for two people with the same BMI, every additional 2ins (5cm) on their waistband added up to a 17% increase in risk for men, and 13% for women.
Professor Elio Riboli, from Imperial College London, said: "We were surprised to see the waist size having such a powerful effect on people's health and premature death.
"There aren't many simple individual characteristics that can increase a person's risk of premature death to this extent, independently from smoking and drinking."
He added: "The good news is that you don't need to take an expensive test and wait ages for the result to assess this aspect of your health - it costs virtually nothing to measure your hip and waist size."
The reason for the link is not entirely clear, but another researcher, Dr Tobias Pischon, from the German Institute of Human Nutrition at Potsdam-Rehbrucke, said that abdominal fat was not like other fat reserves, but could directly influence the development of chronic disease by releasing "messenger substances".
BODY MASS INDEX
Calculated by dividing weight in kilograms by height in metres squared
Normal: 18.5 - 24.9
Overweight: 25 - 29.9
Obese: Above 30
A British Heart Foundation spokesman welcomed the findings, saying they supported previous research which found the risk of heart disease to be higher when fat was concentrated around the waist area.
"It is important a variety of measurements are used to assess body weight and shape. - as well as BMI (Body Mass Index), waist circumference and waist-hip ratio can help to provide a better assessment of health risk.
"If you tend to gather weight around your middle, increasing the amount of activity you do and watching what you eat will help to reduce your risk of heart disease and of dying early."
For 98% of those studied, hip ratio ranged between 0.78 and 1.10 in men and between 0.66 and 0.98 in women.
Within these ranges, each extra 0.1 waist/hip ration meant a third extra risk of death for men, and a quarter extra for women.
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