New figures from the U.S. Census show the four counties of western Massachusetts ranked at the bottom in the state for percentage population growth over the past 10 years.
Hampshire County grew 2.7%, Hampden County grew 0.5%, and Franklin and Berkshire counties lost residents.
Among cities and towns, Holyoke lost the largest number of residents.
Acting Mayor Terry Murphy said he's surprised, but not seriously concerned.
"My concern is to take care of the population that is here and try to make sure that they are getting the services they need and hopefully by doing that, it makes the city more attractive to others," Murphy said.
Pittsfield lost the next largest number of people, followed by North Adams. Mayor Tom Bernard said the decline could have a financial impact on the city.
"When we look at federal program and money that comes in on a proportional or per capita basis, we stand to receive less money and that's a concern," Bernard said.
Bernard said North Adams and the Berkshires need to highlight all they offer potential residents, so that the trend doesn't continue with the 2030 Census.
Some other parts of Massachusetts saw significant population growth.
"Massachusetts overall has grown a little bit more than other Northeastern states, but a lot of this is sort of Boston-centric," said Douglas Hall with the Pioneer Valley Planning Commission.
Hall said the Hispanic population grew by about 30,000 in Hampden, Hampshire and Franklin counties combined. There was also an increase in the Asian population, he said, with fewer whites living in the area and the Black population staying about the same.
Adam Frenier contributed.