More than 300 families are being recruited to take part in a study into whether or not childhood eczema is caused by hard water.
Water softeners will be installed in homes in Nottingham, Leicester, London, Cambridge and the Isle of Wight.
The £1m government-funded survey is thought to be the first of its kind in the world.
The independent trial is being run by Nottingham University which wants to recruit 310 young volunteers.
In a previous study, researchers at the university discovered the condition was 50% more common in primary school pupils living in hard water areas compared with those in soft water areas.
One theory is that hard water contains higher levels of calcium and magnesium, which leads to increased use of soaps which can aggravate eczema.
The homes of the children in the survey will be adapted so all taps will provide soft water apart from one in the kitchen which will supply mains water for drinking.
Professor Hywel Williams, who is running the trial, said: "Patients keep telling me that water softeners help their skin, but other people aren't so sure.
"Carrying out a proper randomised controlled trial will help us find the answer. 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.
"Many patients worry about the possible side effects of the usual treatments for eczema, so this would be a welcome addition to their treatment options."
People with eczema usually experience itchy, dry and flaky skin, which is often red and painful and sometimes weeps or bleeds.
The British Association of Dermatologists said the condition has increased over the past 20 years and currently affects between 15% and 20% of school children.