WASHINGTON (Reuters) -- Half of all young adults in the United States go without health insurance, and more than 15 million Americans were uninsured for four years running, according to a government survey published this week.

The poorest Americans are the least likely to have coverage, and when broken down by ethnic group, Hispanic Americans are most-heavily uninsured, the report from the Agency for Healthcare Research and Quality found.

It said the healthiest people are the most likely to have insurance, but did not say whether their health was a cause or an effect of having insurance.

"During 2002 to 2003, young adults aged 18 to 24 and 25 to 29 were the most likely to be uninsured for at least one month (54.9 and 50.7 percent, respectively)," reads the report, available on the Internet at http://www.meps.ahrq.gov/papers/st123/stat123.pdf.

The 2003 survey found that 24.2 percent of poor Americans under the age of 65, or 3.8 million people, said they went without health insurance for four years in a row. The federal poverty line at the time was an income of $18,400 for a family of four or $8,980 for an individual.

And 6.2 percent of the population under the age of 65, or 15.6 million people, were uninsured for the entire four-year period from 2000 through 2003. At 65 virtually everyone is eligible for Medicare, the federal-state health insurance plan for the elderly.

"Individuals reporting excellent or very good health status were least likely to be uninsured for at least one month during 2002 to 2003 (28.8 and 30.6 percent, respectively)," the report reads.

Other estimates have placed the total number of uninsured Americans at about 46 million, and there is growing concern across the country that over the diminishing number of people who can afford insurance.

The survey found that more than 50 percent of Hispanics said they were uninsured for at least a month in 2002 and 2003, and 15.7 percent said they went without insurance for the whole four years of 2000 through 2003.

The findings are based on the agency's Medical Expenditure Panel Survey, which collects information each year from a sample of U.S. households about health-care use, expenses, access, health status and quality.

On Wednesday, Massachusetts Gov. Mitt Romney signed a bill making the state the first to require residents to have health insurance.