The build up of sticky mucus in the lungs of some cystic fibrosis patients could be prevented using medication, a study has suggested.
The US study of 350 children and young people found denufosol helped keep airways moist and so clear the mucus.
The American Journal of Respiratory and Critical Care Medicine study said those given the drug and not a placebo for six months fared better in lung tests.
The UK's Cystic Fibrosis Trust said the results were "promising".
Cystic Fibrosis (CF) affects over 8,500 people in the UK.
The condition affects the internal organs, especially the lungs and digestive system, by clogging them with thick sticky mucus which makes it hard to breathe and digest food.
Patients are believed to be born with normal lung function, but significant damage is seen early on in life.
In this research, which was funded by Inspire Pharmaceuticals which makes the medication, patients either inhaled denufosol or a dummy version three times a day for 24 weeks, followed by a 24-week period when all received denufosol.
When the study began, virtually all the patients had mildly impaired lung function and were taking drugs to regulate their symptoms.
Denufosol belongs to a class of drugs known as ion channel regulators which help keep the airways moist and aids mucus clearance.
In CF, ion sodium chloride molecules do not flow normally through cell membranes, resulting in thick, sticky mucus which is difficult to cough up.
In addition to causing breathing problems, the mucus is a breeding ground for bacteria and can cause serious respiratory infections.
Currently, breathing exercises or pressure are used to stop the mucus blocking the airways.
The researchers suggest denufosol could help patients whose lung function is still good, or is only mildly impaired.
They say the difference between it and existing drugs is that it targets a cause, rather than symptoms.
Dr Frank Accurso, the director of University of Colorado's CF centre who led the research, said: "Many patients continue to suffer progressive loss of lung function despite treatment of complications.
"Because denufosol can be used early in life, it offers hope for delaying or preventing the progressive changes that lead to irreversible airflow obstruction in CF patients."
A spokeswoman for the UK's Cystic Fibrosis Trust said: "Denufosol is a new type of drug that could delay or prevent lung disease progression in children, teenagers and young adults with CF.
"We have been following the research closely and studies to date have shown promising results in people with CF who have a good lung function.
"Patients could benefit from this new drug in three to four years."