The fire started around 1:50 a.m. in the 7000-block of North Rockwell Street at a West Rogers Park residential apartment complex, according to Chicago police.
Chicago fire officials said not everybody was accounted for.
Three adult men died in the fire, according to Chicago police and fire officials.
Multiple people are missing; three jumped to safety, fire officials said.
CPD said a woman was taken to St. Francis Hospital with unspecified injuries, and was listed in critical condition.
A 17-year-old girl was taken to Lurie Children's Hospital with unspecified injuries in unknown condition.
A third victim, a boy, whose age was unknown, was also taken to Lurie Children's Hospital with unspecified injuries in unknown condition.
A man who lives in the building said he used bed sheets to escape out of his window.
SEE ALSO: Mother reported missing after 'suspicious' fire in her Indiana house: Sheriff's office
"I went to my bedroom, took three sheets and tied them together," Arsenio Duplessis said. "I have metal rail I have outside my window; I tied on there, and I scaled the building."
Residents said they were woken up by fire and smoke in the middle of the night.
Axel Hernandez says he was on his way to work when his mom called to tell him their apartment building was on fire.
He says he immediately returned home.
"I told the firemen, 'I have five cats in there. Can you please take a look?' They found three that passed away. But obviously, the other two are nowhere to be found," Hernandez said.
Fire officials say the back porches were fully engulfed in flames.
All the stairwells had collapsed.
The back apartment units are charred and gutted.
"When the firemen pulled up, there was heavy black smoke coming from all the floors with heavy fire in the rear," Assistant Deputy Fire Commissioner Jim McDonough said.
Reyna Atempa says her nephew and his family live in the building.
She got emotional, as she waited to learn if everyone made it out.
She said in Spanish she had spoken to her nephew two days ago. He had just dropped off his kids at school. And it pains her, not knowing if they are OK.
Nuti Ho and her family were among the people who were rescued by firefighters.
"The smoke started coming. So my dad had to close the door. The smoke continued coming; we were suffocating. And we have to come to the balcony to get some air to breathe, and we were waiting for fire department to come," Ho said.
Fire crews and police investigators have remained at the scene for most of the day.
All the units were occupied.
It's unclear what caused the fire.