This study examines the proposed state constitutional amendment that would change the Illinois state income tax from a flat tax to a graduated income tax. Passage of the constitutional amendment would allow the enactment of Illinois Senate Bill 687, which would raise personal and corporate income tax rates. The highest personal income tax rate would rise by more than 60 percent, from 4.95 percent to 7.99 percent. This would be one of the largest tax increases on small and large businesses imposed in any state in at least the past two decades.