The organisers of a £1m study into whether "hard water" causes eczema in children are looking to recruit more than 300 families across England.
The Government-funded study will focus on hard water areas in Nottingham and Leicester, Cambridge, London and the Isle of Wight.
Researchers will examine whether installing water softeners in homes can help the treatment of childhood eczema.
Some eczema sufferers and doctors have said that living in a hard water area may make eczema worse. They believe hard water contains high levels of calcium and magnesium, leading to increased use of soaps which can act as skin irritants.
The independent trial will be run by the University of Nottingham which is seeking 310 children who suffer from eczema.
Professor Hywel Williams, who is running the trial, said: "I have wanted to do this study for many years as patients keep telling me that water softeners help their skin, but other people aren't so sure.
"If ion-exchange water softeners are found to improve the symptoms of eczema, this will be an extremely important finding for both patients and doctors."
The British Association of Dermatologists said eczema has been increasing over the last 20 years, and affects around 15% to 20% of schoolchildren.