Nearly 40,000 more people left Illinois than moved in between the middle of 2012 and last July, according to new government figures.
The exodus was higher than in any other state.
The numbers get worse when just movement between the states is considered.
Illinois has experienced what is called out-migration for years, but Thursday's data, released by the U.S. Census Bureau, appears to portray a state that is picking up where it left off before the economic downturn, according to a person who has studied Midwest population trends.
Illinois' net loss of 39,562 people between July 1, 2012, and July 1, 2013, far exceeded the next nearest state, Michigan. It saw a net loss of just more than 11,000 people during the same period.
When only movement between the states is considered, Illinois had a net loss of 67,000 people, second worst in the country to New York.
As the economy improves, people are more mobile than they used to be, said Liesl Eathington, an assistant scientist in the economics department at Iowa State University, Ames.
That is hurting some states, including Illinois.
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Iowa saw a net in-migration of 4,814 people between mid-2012 and mid-2013, according to the new figures. But it was helped by the number of people moving to the state from foreign countries. When just movers between the states are considered, Iowa gained 671 people.
Iowa is gaining some of the outflow from Illinois, Eathington said, but Indiana, California and Florida are the greater beneficiaries.
Since 2010, 137,000 more people moved out of Illinois than came to the state. When just movement between the states is considered, that net outflow jumps to nearly 225,000.
Both Iowa and Illinois gained overall population since 2010, but most of it is because of what's called natural increase, or the number of births over deaths.