Small Area Health Insurance Estimates for 2009, released earlier this month, are the only figures measuring insurance coverage for all counties.
A Governing review of county-level SAHIE data found 53 areas throughout the country had more than a third of the population uninsured.
Texas, home to 37 of these counties, has the highest uninsured rate in the United States for those younger than 65 at 26.3 percent.
Texas Gov. Rick Perry has faced criticism for the state's low ranking in recent GOP presidential debates. Perry's supporters attribute the high numbers to the state's presence of illegal immigrants.
Hidalgo County, Texas, was the largest county to rank near the bottom of the list. The county, which borders Mexico, is one of the poorest in the nation.
Massachusetts boasts the highest rate of insured residents at an estimated 95.1 percent of those younger than 65. The state enacted a law in 2006 mandating nearly every resident obtain health coverage, while offering free insurance to some with low incomes.
Of the 20 highest insured rates nationwide, 14 of the counties were in the state.
The SAHIE program is funded by the Centers for Disease Control and Prevention, along with other health groups. Estimates are compiled using census data, food stamp participation, aggregated tax returns, Medicaid records and other data sets.
View data for your area by selecting a state and county below. Please note that all estimates are for the population younger than 65.
Further breakdowns of health insurance estimates can be obtained from the Census Bureau.
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