Scientists from King's College, London, said the link between asthma and obesity could become a vicious circle because some parents are reluctant to let their children exercise if they are asthmatic.
"Many parents believe that a child with asthma should not exercise. Thus, asthma may induce a sedentary lifestyle in asthmatic subjects which could result in an increase in fatness," said researchers.
They added that children who exercised less could suffer more as their airways narrow.
The researchers studied 15,000 children aged between 4 and 11 and found that 17 out of every 100 children were suffering from asthma.
Exercise levels
The study, published in Thorax, found that the heaviest children were most likely to be at risk. They found that among the inner-city children that overweight girls were more at risk than overweight boys and thought this could be linked to hormone levels.
But Dr Martyn Partridge, chief medical adviser for the National Asthma Campaign, said asthma is not limited to overweight children and that some could become fat because they weren't exercising.
"This study and several others suggest that there may be a greater number of obese children with asthma, but it is impossible to say whether they are obese as a result of the disease (possibly due to less exercise) or whether their obesity has caused their asthma."
He said that if obesity caused asthma, improving the child's nutrition could help.
"We know for example that daily fruit can reduce the risk of lung disease," he said.
Dr John Harvey, of the British Thoracic Society, said he would like to see "more research".
"Asthma can affect people of all ages and all sizes. Although obesity has been linked with asthma, health professionals must treat all patients according to correct measurement of their lung function and symptoms.
"More research needs to be conducted in this area," he said.