Mild asthma and severe asthma may be two different diseases, say scientists.
Over five million people in the UK have asthma
Their theory may explain why people with severe asthma do not respond well to existing treatments.
Their findings, published in the European Respiratory Journal, are based on a study of more than 300 patients from nine countries.
The researchers said their findings could boost efforts to find better drugs and treatments for people with severe asthma.
People with mild asthma generally respond quite well to treatment. However, the same cannot be said of people with severe asthma.
They are more likely to suffer attacks and to need emergency medical care.
Survival rates for very severe asthma are poor and are even similar to several types of cancer.
Scientists from across Europe have worked together to try to discover more about severe asthma.
The study, which was partly funded by the European Union, looked at 158 patients with mild to moderate asthma and 163 patients with severe asthma.
All of those with mild or moderate asthma were able to control their symptoms with low doses of inhaled corticosteroids. These drugs are routinely prescribed for people with asthma.
However, very few of those with severe asthma were able to keep their symptoms under control.
They were much more likely to have asthma attacks and their ability to do day-to-day activities was reduced.
This was despite the fact that they were all taking much higher doses of corticosteroids and one in three were given powerful steroids.
The researchers also carried out blood tests on all of the patients. They found that both groups had very different levels of key chemicals in their blood.
This led the researchers to conclude that mild asthma and severe asthma are very different and may even be two different diseases.
Women affected more
In addition, the study found that mild and severe asthma affect men and women differently.
They discovered that slightly more men had mild asthma compared to women, with a ratio of 1.6:1.
However, women were four times more likely to have severe asthma compared to men.
The scientists said this finding highlighted the fact that severe asthma is unique and that much more research is needed to find out what causes it.
The UK's National Asthma Campaign said further research is needed.
"It is likely that there are several mechanisms responsible for increased asthma severity and it may be that different mechanisms operate in different groups of people," said a spokeswoman.
"It is unclear whether severe asthma is at one end of a spectrum with mild-moderate asthma, or whether it is in fact a different condition with different inflammatory processes.
"Whilst this is an interesting study, further research is needed to achieve greater understanding of these mechanisms."