The victims, all in their 70s, were transported to Stroger and St. Mary hospitals to be treated for smoke inhalation, authorities said.
The fire broke out at about 2 a.m. in the 4300-block of West Thomas Street, police said.
"They all family in there," a witness said. "They are like family members everyone that lives there."
Debris from the fire could be seen piled on the ground as neighbors look inside the partially charred building.
Fire officials said it all happened inside the multi-unit building...near North Kildare Avenue...around 2 this morning.
"We helped four individuals leave the building, two individuals self-evacuated on their own," CFD Deputy Chief Ernie Pinkston said.
The fire began in the building's basement, but didn't spread to other floors.
Firefighters did have to smash windows on the first and second floors to help smoke escape the building.
Hours later, plywood covers the building's windows.
A neighbor who lives across the street and said he was awoken to the frightening moments this morning.
"I came and look out and see if they was fine and then I seen the ambulance was here and the firetruck was here and they pulling out the people that lived here," the witness said.
The cause of the fire is under investigation.