Scott County's population bounced upward by more than 2,200 people between 2007 and 2008, according to government estimates.
The spike in population is the strongest the county has seen in years, and it's being driven in part by people who have moved from other parts of the United States.
The news wasn't so good for Rock Island County, which returned to the negative column after a one-year gain.
The U.S. Census Bureau released population estimates for the country's 3,100 counties and 363 metropolitan areas two weeks ago.
The Quad-Cities population grew to 377,625 on July 1, 2008, a 1,987 person increase from the previous July. Scott County in Iowa and Rock Island, Mercer and Henry counties in Illinois make up the Quad-City metro area.
The boost didn't keep the area from slipping a spot in the nation's ranking of metro areas, however, as other places grew at a faster rate.
The Quad-Cities ranked 133rd last year, down from 132nd the year before.
The area's growth was mostly confined to Scott County, according to the census data. The county's increase of 2,245 people, or 1.4 percent, put its population at 164,690.
The previous one-year growth increase for the decade was 1,375 in 2006.
"It seems like a pretty good growth rate for one year," said Tim Huey, planning and development director for Scott County.
Demographers say big one-year changes should be viewed with some caution. And Huey noted the gains could be a blip.
Previous one-year gains have been more modest. Since 2000, the county's population has grown by only 3.8 percent.
Half of last year's estimated increase came from what's called natural increase, or the number of births over deaths.
The government also estimated, however, that 871 people moved to Scott County from other places in the U.S. That's a departure from the previous year when the county lost 350 people to other U.S. counties, according to government estimates.
In Rock Island County, the Census Bureau estimated population at 146,886 as of last July, a 250-person decline from 2007. That's a reversal from the previous year, when the government reported the county gained 757 people, according to revised figures.
The county saw an increase in births over deaths and migrants from other U.S. locations. However, it also saw 1,160 people leave last year, according to the data.
Typically, people in the Quad-Cities who do move go between Scott and Rock Island counties rather than some other place in the U.S. That could explain some of the shift in population in the two counties.
The census data, though, did not define where people moved to or came from.
Area real estate officials say they have not noticed a marked shift, although there was a change in where new home and condo sales occurred last year.
Eighty-seven percent of sales were on the Iowa side, according to figures supplied by Ruhl and Ruhl Realtors. The year before, it was 64 percent. Typically, 80 percent of new sales are in Iowa, said Dave Falk, the firm's director of new construction and development.
He said the change last year was the result of less available housing stock in Rock Island County.
"The inventory is down considerably," he said.