Most of that growth has been in the Washington suburbs, which added 515,000 people from 1990 to last year, a number nearly equal to Washington's current population.
The new population figures are part of the Census Bureau's 1999 population estimates for the nation's major metropolitan regions. They are based on the 1990 census, updated with data from birth and death records, building permits and tax returns.
They are the last population estimates before the release, beginning in December, of head counts taken in the spring in the 2000 census.
To the Census Bureau, the Baltimore-Washington region is known as the Washington-Baltimore Consolidated Metropolitan Statistical Area, or CMSA.
Designated in 1993, it is the fourth-largest CMSA in the nation, after New York, Los Angeles and Chicago and their surrounding regions.
It is a sprawling piece of real estate composed of densely settled suburban counties and core cities from Harford County to Northern Virginia. They're bound together by economic and commuting ties.
The three subregions include:
Baltimore and six nearby Maryland counties.
Hagerstown and the rest of Washington County in Maryland.
Washington, 19 surrounding counties and six independent cities in Maryland, Virginia and West Virginia.
CMSA data are used by government agencies for the allocation of funding and services and the setting of reimbursement rates. Private industry uses the data for decisions on marketing and the siting of plants and corporate headquarters.
The new data show that the Washington-Baltimore region's population grew from 6.73 million to 7.36 million, or 9.4 percent, between the 1990 census and the 1999 estimates just released. That closely matches the population growth of the nation during that period and is just short of the 10.1 percent growth in those portions of the nation included in metropolitan areas.
Most of the growth has occurred in the Washington metropolitan area, which grew by an estimated 517,169 people, or 12.2 percent, during the 1990s. That brought the Washington area's total to 4.74 million people.
The Baltimore region, meanwhile, had a net gain of 109,082 people, or 4.6 percent. The Baltimore area's population last year was estimated at 2.49 million.
Within those two metropolitan areas, all of the growth has been outside the central cities that gave them birth. Baltimore and Washington both shrank considerably in population during the 1990s.
Baltimore lost 103,333 residents, or 14 percent. That cut its population to 632,681 last year and moved the city from the nation's 12th largest in 1990 to 16th.
Washington shrank by 87,900 people, or 14.5 percent, slipping to 519,000. The capital lost its rank as the 19th largest city in 1990, sliding to 24th last year.
Of the 224 cities in the nation with populations greater than 100,000, Washington was the second-fastest-shrinking city in the 1990s, and Baltimore was the third. Only St. Louis lost population at a faster rate, according to the Census Bureau.
In absolute numbers, only Philadelphia lost more people than Baltimore during the 1990s, shrinking by 167,976 people, or 10.6 percent.
In the Washington-Baltimore area, Hagerstown and Fredericksburg, Va., also recorded population losses during the 1990s.
But the region's suburban counties grew briskly.
In Maryland, the metropolitan suburbs swelled to 77.8 percent of the state's total population last year, up from 74.6 percent in 1990. That makes it the second-most-suburban state after New Jersey.
The central cities, meanwhile, shrank from 18.2 percent of the population to 14.9 percent. The rural, or "non-metropolitan," population of Maryland remained little changed, at 7.3 percent of the population.
In the Baltimore metropolitan area, Howard County grew fastest in numbers of people and as a percentage. The county added 55,784 residents, a gain of 29.8 percent. The population last year was 243,112.
Other suburban gains in the area during the 1990s included:
Anne Arundel County, up 53,244, or 12.5 percent.
Harford County, up 35,776, or 9.6 percent.
Baltimore County, up 31,780, or 4.6 percent.
Carroll County, up 29,096, or 23.6 percent.
Queen Anne's County, up 6,735, or 19.8 percent.
Nationally, the fastest metropolitan growth rates were generally in the South and West. The highest rates of population loss were mostly in northern states from Maine to North Dakota.
Among cities, the fastest growth rate in the 1990s was recorded in Henderson, Nev. The booming Las Vegas suburb grew 155 percent, from 65,109 to 166,399. The second-fastest-growing city was another Las Vegas suburb, and the sixth was Las Vegas.
Among metropolitan areas, the highest population growth rates during the decade were in the Las Vegas area and in the Laredo and McAllen-Edinburg areas in southern Texas near the Mexican border.
There were no changes between 1990 and 1999 in the top 10 rankings for the nation's most populous metropolitan areas, but Atlanta climbed from 13th place to 11th, displacing the Miami-Fort Lauderdale area and the Seattle-Tacoma-Bremerton, Wash., region. The Phoenix region climbed from 19th to 14th.
The highest rates of population loss for metropolitan areas during the 1990s were in Grand Forks, N.D., Utica-Rome, N.Y., and Steubenville, Ohio.