Carroll's unemployment rate rose to 5.7 percent in August from 5.4 percent in July, leaving 203 more people out of work in the county.
A total of 3,762 people received unemployment benefits in Carroll during August, compared with 3,559 in July, the Department of Economic and Employment Development reported.
But officials say they're unsure why the numbers went up.
The increase came just after both the county and state unemployment rates had dropped to their lowest points since November 1991.
"We had a lot of people coming in and filing, but it's not the result of any one industry," said Theodora Stephen, manager of the DEED office in Westminster.
"[August] is not normally a month for anything to increase," she said. "We were surprised that it went up."
Typically, construction workers are still employed and students continue to work at their summer jobs through August, she said.
"It's just the recession," Ms. Stephen said.
The county numbers are nearly a full percentage point higher than the unemployment rate of 4.8 percent logged in Carroll in August 1991.
Meanwhile, state unemployment figures increased only slightly from 6.5 percent in July to 6.6 percent in August, resulting in 175,565 people receiving benefits across the state.
Statewide figures ranged from a high of 10.7 percent unemployment in Baltimore City to a low of 3.6 percent in Montgomery County.
Earlier this year, county unemployment had risen from 5.7 percent in May to 6.1 percent in June, while state figures rose from 6.7 percent in May to 6.9 percent in June.
State unemployment was recorded at 5.8 percent in August 1991.
The percentages reflect unemployment within the civilian labor force, which is defined as the total number of people employed added to those who are actively looking for work.
In Carroll, the civilian labor force dropped from 66,696 in July to 65,913 in August.
The state's labor force figures dropped from 2,716,535 in July to 2,675,560 in August.
Nationwide, unemployment figures dropped from 7.6 percent in August to 7.5 percent in September.
U.S. numbers are one month ahead of those compiled for the state.