Six fire engines and around 40 firefighters were called to a fire at a hostel on Wentworth Street in Whitechapel.
Part of a flat on the first floor of the building was damaged by fire
Station Commander Tom Ronan, who was at the scene, said: "One man left the building before the Brigade arrived and was treated on scene by London Ambulance Service crews. Firefighters carried out a systematic search of the building to ensure no one was still inside.
"Crews worked quickly to bring the fire under control and damp down remaining hot spots."
The fire was believed to have been accidental and caused by a discarded cigarette.
The Brigade was called at 0813 and the fire was under control by 0857. Fire crews from Whitechapel, Shadwell, Dowgate, Bethnal Green, Dockhead and Islington fire stations attended the scene.