A second young girl escaped injury when her mother dropped her from a second-floor window into a firefighter's arms.
No names were released.
Denver Fire spokesman Phil Champagne said bystanders reported people were trapped inside after the blaze started around 9 a.m. Sunday in a second-floor apartment of a three-story building.
The 3-year-old girl and her mother were taken to Denver Health Medical Center. Their conditions were unknown. Firefighter Jason Jones said the girl wasn't able to breathe on her own, but she had a pulse.
Another mother and daughter were trapped in the next apartment.
``I noticed a commotion in the alley,'' fire engineer Mark Tenpas said. ``I looked up to see a little girl dangling from a window.''
Tenpas said the mother was holding a girl between 4 and 8 years old by her arms outside the window. Tenpas told her to hang on, but the mother said, ``I can't hold her anymore.''
The girl dropped onto Tenpas and a teenage boy.
``She was totally calm,'' Tenpas said. ``I handed her off right away.''
Firefighters led the girl's mother outside to safety, but she needed medical attention.
Champagne said the fire damage was confined to one unit. Other units sustained heavy smoke damage.
The cause of the fire was unknown, but witnesses reported hearing screaming and a loud crash before the fire erupted.