The Centers for Disease Control and Prevention, which conducted the survey in 2003, said it was the first study to offer a state-by-state look at the prevalence of smoking in American homes.
Utah led the nation, with people in nearly nine out of 10 homes saying smoking was never allowed. The state's large population of Mormons, who eschew tobacco, probably contributed to that statistic, the agency said.
Kentucky was in last place, with a little more than half of households sending smokers outside (or, at least, to the garage).
But even in Kentucky, smokers found fewer places to light up. Ten years earlier, only a quarter of the state's households barred smoking.
''That really says that people are starting to understand the hazards of secondhand smoke,'' said Dr. Corinne Husten, co-author of the study and chief of the epidemiology branch of the CDC's Office on Smoking and Health.
Tobin Hilliard joined the millions of Americans living in smoke-free homes when he moved in with his non-smoking fiancee 10 years ago. He had to abandon the pleasure of smoking a cigarette at the kitchen table, on the living room couch or in the bedroom.
''It was just understood: 'If you're lighting up, you will be stepping out into whatever the weather conditions are,' '' said Hilliard, 35, who is still a pack-a-day smoker in Clermont, Fla.
The CDC report was based on a national survey done mostly by telephone every two years. For a household to be included in the results, everyone 15 and older had to respond, and they all had to agree on the smoking rules.
The survey covered 127,000 U.S. households in 2003, the most recent year for which such data was available. The study looked at 900 to 7,000 homes in each state. Similar numbers were surveyed in previous years.
Participants were asked whether smoking was allowed everywhere in the home, only in some places or not at all.
Among households with at least one smoker, the national prevalence of take-it-outside rules rose from about 10 percent in the early 1990s to 32 percent in 2003. Among households with no smokers, the percentage rose from 57 percent to almost 84 percent.
The CDC said the increases were driven in part by scientific reports and other information in the last 15 years warning that secondhand smoke causes premature death and disease.
A growing number of state and local smoking bans in restaurants, bars and workplaces may also have been influential at home, Husten said.
Loyd Silberstein, a retired school teacher in California, said he smokes at home - but not when his children or grandchildren come over. On those occasions, he goes out to the backyard or garage.
''My wife says I don't care about her, just the kids,'' laughed Silberstein, 75, of San Mateo.
The study was published the CDC's Morbidity and Mortality Weekly Report.
In another MMWR article this week, surveys of nearly 750,000 teens in 137 countries and territories showed that students exposed to smoking at home were most likely to take up the habit themselves.
The study found that more than 71 percent of nonsmoking students surveyed in Europe said they were exposed to cigarette smoke at home. The exposure was much lower in other parts of the world - particularly in Africa, where the statistic was just 23 percent.