There was a sharp fall in the number of children admitted to hospital with severe asthma after smoke-free legislation was introduced in England, say researchers.
A study showed a 12% drop in the first year after the law to stop smoking in enclosed public places came into force.
The authors say there is growing evidence that many people are opting for smoke-free homes as well.
Asthma UK says the findings are "encouraging".
Researchers at Imperial College in London looked at NHS figures going back to April 2002.
Presenting their findings in the journal Pediatrics, they said the number of children admitted to hospital with severe asthma attacks was rising by more than 2% a year before the restrictions were introduced in July 2007.
Taking that into account, they calculated the fall in admissions in the next 12 months was 12%, and a further 3% in each of the following two years. They say over the three-year period, this was equivalent of about 6,800 admissions.
The fall was seen among boys and girls of all ages, across wealthy and deprived neighbourhoods, in cities and in rural areas.
Prior to the smoke-free law much of the debate on the legislation centred on protection of bar workers from passive smoke.
At the time many critics said smokers would respond by lighting up more at home - harming the health of their families. But the authors of this study say there is growing evidence that more people are insisting on smoke-free homes.
The lead researcher, Prof Christopher Millett, said the legislation has prompted unexpected, but very welcome, changes in behaviour.
"We increasingly think it's because people are adopting smoke-free homes when these smoke-free laws are introduced and this is because they see the benefits of smoke-free laws in public places such as restaurants and they increasingly want to adopt them in their home.
"This benefits children because they're less likely to be exposed to second hand smoke."
These findings reinforce evidence on the impact of smoke-free legislation from studies in North America and Scotland, which also showed a fall in hospital admissions for children with severe asthma attacks. The law in England has also resulted in fewer admissions for heart attack.
Emily Humphreys from the health charity, Asthma UK, welcomed the findings: "This is something we campaigned for, so it is particularly encouraging that there has been a fall in children's hospital admissions for asthma since its introduction.
"We have long known that smoking and second hand smoke are harmful - they not only trigger asthma attacks which put children in hospital but can even cause them to develop the condition."
She said the need now was to do more to prevent children and young people from taking up smoking, and she repeated the charity's call for the introduction of plain packaging for tobacco.