Chicago Heights police said they responded to a call about a disturbance in the 1500 block of Hanover Street just before 4 p.m.
Officers found two boys, 14 and 13 years old, with apparent stab wounds. Both were taken to a local hospital.
The 14-year-old was later transferred to Stroger Hospital and the 13-year-old was transferred to Comer Children's Hospital.
Chicago Heights police and Illinois State Police Crime Scene Services are investigating.
Chicago Heights police asked anyone with information to contact them at 708-756-6400.