CPD said the shooting happened in the Humboldt Park neighborhood's 3200-block of West Division Street just after 7 p.m.
An 8-year-old boy found a gun in the home and shot another boy, 7, police said.
The 7-year-old, shot in the arm, was transported to Stroger Hospital in serious condition, CPD said.
Investigators recovered a gun from the scene.
No one is in custody, and Area Five detectives are investigating.
Further information was not immediately available.
This is a developing story. Check back with ABC7 for updates.