People can be obese yet physically healthy and fit and at no greater risk of heart disease or cancer than normal weight people, say researchers.
The key is being "metabolically fit", meaning no high blood pressure, cholesterol or raised blood sugar, and exercising, according to experts.
Looking at data from over 43,000 US people they found that being overweight per se did not pose a big health risk.
The results are published in the European Heart Journal.
In the study at the University of South Carolina, more than a third of the participants were obese.
Of these 18,500, half were assessed as metabolically healthy after a physical examination and lab tests.
This subset of metabolically healthy obese people who did not suffer from conditions such as diabetes, high cholesterol or high blood pressure, were generally fitter and exercised more than the other obese people.
And their risk of developing or dying from cardiovascular disease or cancer was identical to people of ideal weight and was half that of "metabolically less fit" obese people.
Lead researcher Dr Francisco Ortega, who currently works at the University of Granada in Spain, said the findings show that getting more exercise can keep you healthier, even if you still carry a bit of extra weight.
"This research highlights once again the important role of physical fitness as a health marker."
Most of the men and women in the study came from a similar background, meaning the results may not apply to everyone. They were mostly Caucasian, well educated, and worked in executive or professional positions.
Amy Thompson, of the British Heart Foundation, said: "In the majority of cases, obesity is an undeniable risk factor for developing coronary heart disease. However, these studies remind us that it is not always your weight that's important, but where you carry fat and also how it affects your health and fitness.
"It is particularly important to be aware of your weight if you are carrying excess fat around your middle. The fat cells here are really active, producing toxic substances that cause damage which can lead to heart disease.
"Maintaining a healthy diet with lots of physical activity can help to slim you down as well as reduce your risk of heart health problems.
"But don't get too caught up on the numbers on the scale. Calculating your body mass index and measuring your waist are great ways to keep on track. If you are concerned about your weight and want to make changes to your lifestyle, make an appointment with your GP to talk it through."