Loudoun and Stafford were among the fastest-growing counties in the nation last year, according to Census Bureau figures that underscore the rapid expansion of the Washington region's far-flung suburbs.

The metropolitan area continues to grow at a pace that puts it on track this decade to exceed its 16 percent increase in the 1990s, though the rate of change slowed slightly last year, according to the census estimates to be released today.

Increasingly, census figures show, the area's growth is sprawling 35 miles or more from the District, on former farms where townhouses and mini-mansions now sprout. Two-thirds of the region's increase last year was in its outermost suburbs, the census numbers show, with the sharpest growth rates in Northern Virginia.

For the second consecutive year, Loudoun was the second-fastest-growing county in the nation. Its population increased 7.3 percent last year. Stafford County's 6.2 percent increase pushed it into ninth place among the nation's 3,141 counties. The Washington area was surpassed only by metropolitan Atlanta, which had three of the nation's 10 fastest-growing counties.

The nation's top-gaining county last year was Rockwall, an eastern Dallas suburb that grew 7.9 percent last year, to a population of 50,858.

The population boom in Loudoun and Stafford is fed by a steady stream of people moving both from closer-in neighborhoods and from other parts of the country. They have sought bigger homes and more open space, but in recent years they also have elected slow-growth county boards of supervisors to fight traffic jams and other troubles caused by rapid change.

The census estimates, which are for the year ending last July 1, said the region added nearly 98,000 people, either arrivals or newborns, over the previous year. That means more than 5.6 million people live in the Washington area, which stretches from southern Maryland to Virginia horse country, and from the Catoctin Mountains to the Chesapeake Bay.

Experts said the region's growth probably cooled slightly last year because a weakened economy made people cautious about moving and produced fewer high-paying professional jobs to draw them here. Even so, the D.C. region was the only large metropolitan area to gain jobs last year, and federal defense spending will cushion its unemployment rate. Regional planners predict steady growth.

It is striking, experts say, that despite the recent focus on reducing sprawl, today's suburbs are exploding outward much as their predecessors did in the 1950s. "It's going to take years to make the turn toward the alternative development pattern that everybody talks about," said Robert E. Lang, director of the Metropolitan Institute at Virginia Tech.

Still, Lang points out, suburbanization has changed in some ways. Instead of working in the city as 1950s suburban residents typically did, the newest suburbanites often commute to the older suburbs, such as Fairfax or Montgomery counties. And the average home today is larger, with one in four Stafford and Loudoun homes having nine or more rooms.

Loudoun County expanded in 2002 at seven times the national average, even though its growth rate receded slightly from the year before, according to census figures. The county board passed restrictions on new development that took effect in January, but thousands of housing units were already in the development pipeline.

Growth accelerated last year in Stafford County, which is part of a cluster of jurisdictions off Interstate 95 near the city of Fredericksburg. That cluster includes Spotsylvania County, whose growth rate ranked 13th in the nation last year.

"The 95 corridor was ignored in the 1990s," when development filled up land off I-66 west of the city, said Stephen Fuller, a regional economist at George Mason University. "Now all of a sudden, Stafford County is not that far out."

Stafford, which a decade ago was primarily dairy farms and agricultural fields, now is dotted -- and in some places, plastered -- with housing subdivisions, big box stores and mini-malls. According to the county's Planning Department, 843 acres of agricultural land was approved for housing development in 1998. In 2002, that rose to 10,587 acres.

But, much as "Californicate" has become an epithet in some Western states, the term "Fairfaxed" has sprung up to describe the growth-induced sky-high taxes, endless pavement and traffic that residents of newer communities fear.

Stafford County is considering impact fees that would require developers to pay for road upgrades, and rewards for landowners who voluntarily downzone their property. The county and its neighbors also have tried to promote a pickier philosophy of economic development that regulates more tightly how new homes look, and steers the most attractive businesses to the most highly traveled roads.

"I think the county wants a new image now," said Darrell Painter of Dumfries, who last November opened a business that sells trailers and trucking equipment. "They want to limit noise and sprawl. They want a more upscale image."

Painter initially wanted to locate his business nearer to the Capital Beltway, but he realized that Stafford was jumping with opportunity. "The amount of booming is amazing," he said.

But the changes unsettle longtime Stafford residents Mary Jo and Tom Behm, who moved to the county 29 years ago and have raised five children there. They leave in the morning darkness for jobs at the Federal Emergency Management Agency in Washington, making the nearly three-hour round trip by commuter train because it is less stressful than driving the clogged roads.

They no longer do errands on weekends -- too much traffic -- and generally travel on back roads. They are concerned about growth's impact on the county schools. "As soon as they build one school to relieve overcrowding, it's already too small -- they can't keep up," said Mary Jo Behm, 50.

"We came here because it was a rural area, a small town," said Tom Behm, 54. "Now we're looking toward retirement, and we're already looking for another small town."

Aside from Stafford, the only counties in the region that grew faster last year than the year before are Calvert, Frederick and St. Mary's. The census figures said growth was flat in the close-in suburbs of Alexandria and Arlington, though that is disputed by state population estimates that say those suburbs grew. The census earlier announced a small decline in the District. The Census Bureau derives its estimates from government records such as birth certificates, visa data, Medicare enrollment records and tax returns, making them less accurate than the door-to-door count conducted every 10 years.

Stafford County, once devoted largely to agriculture, now is booming with housing and other developments.