Scientists believe they have found what triggers many children with eczema to go on to develop asthma.
The Public Library of Science Biology study points to a way to stop what is known as the "atopic march".
The US team at the Washington University School of Medicine showed that a substance made by the damaged skin triggered asthma symptoms in mice.
The same substance, thymic stromal lymphopoietin (TSLP), is also produced in the lungs of asthma patients.
Early treatment of the skin rash and blocking TSLP production might stop asthma developing in young patients with eczema, they hope.
Drugs that act on TSLP might also protect against asthma development even in cases that are not linked to eczema.
Allergies and asthma often occur together. Studies show that 50-70% of children with severe allergic skin problems - atopic dermatitis - go on to develop asthma.
The researchers studied mice bred with a genetic defect that made them develop a condition similar to eczema in humans.
The defective skin secreted TSLP, which the researchers believe alerts the body that its protective barrier has failed.
When they tested the lungs of the mice, they found this tissue also responded strongly to the TSLP signal and had the hallmark traits of asthma - mucous secretion, airway muscle contraction and invasion of white blood cells.
They did more experiments and found that even mice with normal skin but bred to overproduce TSLP also developed asthma-like symptoms, suggesting TSLP is indeed the culprit.
Lead researcher Dr Raphael Kopan said: "We are excited because we've narrowed down the problem of atopic march to one molecule.
"We've shown that the skin can act as a signalling organ and drive allergic inflammation in the lung by releasing TSLP.
"Now it will be important to address how to prevent defective skin from producing TSLP. If that can be done, the link between eczema and asthma could be broken."
Dr Elaine Vickers of Asthma UK said: "This is the first piece of research to suggest that the natural protein TSLP could play a direct role in causing people with eczema to develop asthma.
"These results were obtained from studies with mice, so it is important to establish whether the same causal link exists in humans.
"Scientists are already exploring the potential of targeting TSLP to create new treatments for eczema, asthma and other allergic conditions.
"Although it is still a long way off, this research raises the exciting possibility that as well as improving symptoms, these treatments might be able to limit, or even prevent, the development of asthma."