To be sure, advantage goes to sunny climes. Counties in the five states with the biggest overall population gains since 2000 -- California, Texas, Florida, Georgia and Arizona -- accounted for well over half of the 100 hot counties.
Add in Nevada, the fastest-growing state, and six Western and Southern states accounted for two-thirds of the top 100's population growth, according to data prepared for American Demographics by ESRI, a supplier of geographic information-systems software and data.
A supersize county
Just one supersize county -- Southern California's Riverside County, bigger than Connecticut and Rhode Island combined -- has added 474,000 residents since 2000.
But even slow-growing regions have pockets of growth. Since 2000, the Midwest's population has grown by just 2.7% (1.8 million residents), yet the region is home to 14 of the 100 fastest-growing counties.
Case in point: Ohio's population has grown about 1% since 2000, one of the lowest growth rates. Yet Ohio is home to two of the 100 fastest-growing counties: Delaware, north of Columbus, and Warren, northeast of Cincinnati.
In Delaware County, cornfields have given way to Polaris Fashion Place, a massive mall that opened in 2001. Cincinnati's Procter & Gamble Co. in 1995 opened a research center in Warren County, helping fuel growth. The two counties accounted for more than three-fourths of Ohio's population gain this decade.
Similarly, Illinois' population has grown 3% since 2000, yet three counties near Chicago -- Kendall, Kane and Will -- are on the top-100 list with strong double-digit growth.
Kane and Will are among 11 counties nationally with population gains above 100,000 since 2000. Kendall's population has rocketed 49% since then, making it the ninth-fastest-growing county. Like many high-growth counties, Kendall has a small base: Its current population (81,000) is a fraction of the more than 5 million in Chicago's Cook County.
The most sizzling growth has occurred under the sun. Flagler County, Fla., on the coast between Daytona Beach and Jacksonville, ranks No. 1, with a gain of 75% since 2000. It boasts wide-open beaches, tranquil surroundings and plenty of golf. The local tourism authority proclaims: "'Peaceful' is not just a word; it's a lifestyle."
The fastest-growing counties tend to be on the edges of major metro areas. Four of the top 10 counties surround Atlanta. Loudoun County, Va., outside Washington, is No. 2 on the list.
"It's supply and demand," said Edmond Ting, a manager-data development at ESRI. "There is the demand for housing, and developers are just looking for cheaper land to put in a lot of housing."
Relative home values
Cheap is relative. The median home value in Sabine, Texas, the lowest-priced place in the top 100, is about $64,000. The median home value in northern California's Placer County, the most expensive, is $587,000. Home prices in the fastest-growing county, Florida's Flagler, are rising 12% a year.
The recent end of this decade's unprecedented housing boom will mean less construction, slower growth and possibly lower prices in some overheated markets. But there's no denying how prices have surged. California and Florida, which are first and third in population growth since 2000, are home to all 25 markets with the greatest percentages of housing appreciation this decade, according to government data.
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Mya Frazier and Jack Neff contributed to this report.