ATLANTA — Smoking is forbidden in nearly three out of four U.S. households, a dramatic increase from the 43 percent of homes that prohibited smoking a decade ago, the federal government reported Thursday.

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.

The report is based on a national survey done mostly by telephone. 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 with such rules rose from 57 percent to almost 84 percent.