Firefighters were called just before 8 p.m. Friday night to the 100-block of South Laramie in the South Austin neighborhood.
Firefighters say they removed two boys and one adult from the burning building. A four-year-old and the adult are in critical condition.
"The fire's still under investigation but it appears the main body of fire was on the rear porches of both buildings," said Don Hroma, Chicago Fire Department.
A Chicago police officer was treated for what may be asthma-related issues.
Firefighters say a number of people have been displaced by the fire.