If you are overweight before you start school it will be very hard to lose that extra weight once you get older.
That's according to research which has shown that how overweight you are is determined when you are very young.
The study by medical experts in Plymouth shows one in four kids in England are obese. That means their weight is dangerous to their health.
And experts reckon that the problem is getting worse, as kids are fatter than they were 25 years ago.
The health experts behind the study say that we need to take a new approach to keeping fit.
At the moment lots of attention is put on making sure children are healthy while they are at school, but that might not be as useful as we think.
Instead, the experts want children to eat more healthily and take lots of exercise before they start primary school.
Too much food
What you weigh when you're five years old has an effect on what you will weigh when you're nine, according to the study, and it seems girls are more likely to gain weight at an early age than boys.
Experts reckon the extra weight gained is because of too much food and not enough activity before you go to school.