A fire department spokesperson says the victims were trapped in the building's attic. There was only one way in and out.
It appears the fire started on the third floor.
Everyone else was able to get out of the building safely. Firefighters discovered the bodies of a 20-year-old man and a 40-year-old man when they searched the building.
"It was unfortunate the occupants didn't have a smoke detector, early warning device up there. Otherwise, we'd maybe have a different story here," said Curtis Powell, deputy district chief, Chicago Fire Department.
Chicago firefighters was set to pass out smoke detectors Tuesday.
The cause of the fire has not been determined.