Scientists are investigating if installing a home water softener can relieve children's eczema symptoms.
The Department of Health-backed study will also look at the differing effects of hard and soft water on the condition which causes dry and itchy skin.
Eczema has been reported to be more common in hard-water areas but it is not understood why that might be.
One mother who has taken part in the research said she had seen a "dramatic improvement" in her son's condition.
Eczema affects up to 20% of children in the UK.
It typically occurs on the face, neck, and the insides of the elbows, knees, and ankles.
In infants, it is usually seen on the forehead, cheeks, forearms, legs, scalp, and neck.
The study, which began in 2007 and will run until this summer, is recruiting children aged six months to 16 years with moderate to severe eczema.
Eight research centres in Portsmouth, Boston, Lincoln, Nottingham, Leicester, Cambridge, Newport on the Isle of Wight and London are involved.
Just over 230 children have already taken part, but the researchers are looking for another 80.
The children, or their parents, will be asked to complete a daily diary to record eczema symptoms and will be issued with a small computerised wristband to monitor their level of night-scratching while they are asleep.
The families have the water softener for 12 weeks, then it is taken away from another four to see if there is any change.
The children continue to use their normal eczema treatment throughout the study.
Anne-Marie Crawford-Flanagan from Portsmouth has just finished taking part in the trial with her 15-month-old son Dylan.
He first developed eczema when he was about three months old, and his symptoms became increasingly serious.
"After a few months, he developed atopic eczema where his skin blistered. It was horrendous."
Every day, Anne-Marie had to cover Dylan's body in cream and then put a wetted outfit of special tights and a turtle-neck long-sleeved top with mittens, plus a dry version of the outfit on him to try and stop him scratching.
Each night, he had to bathe in emollient bath oils - before the whole process was repeated.
She says: "Dylan had bleeding sores, and even changing him was heart-breaking. Every time his skin was exposed, he started scratching.
"And he wouldn't sleep for more than an hour without waking up to scratch.
"We had about a dozen different creams on prescription and I was at my wit's end."
But using the softener led to rapid and significant benefits.
"Less than two weeks after the softener was installed there was a dramatic improvement. Now there's barely a mark on him.
"He's a completely different baby."
When the softener was removed, Dylan's eczema started to return, so the family have now bought their own device.
"He has been sleeping through, and he doesn't have to wear the special suits any more. Now he's just in normal vests."
Professor Tara Dean of Portsmouth University, one of the experts co-ordinating the study, said: "If water softeners are found to improve the symptoms of eczema it will be a breakthrough for both patients and doctors."
Margaret Cox, chief executive of the National Eczema Society, said she looked forward to having the evidence to be able to advise families if using a softener would help.
"Anecdotally there are years of reports of people saying soft or softened water was helpful."
She said the suggestion was soft water was better to wash with, and therefore needed less soap or detergent - which are irritants for people with eczema.