Four fire engines and 21 firefighters and officers were called to a fire at a block of flats in Beaconsfield Road, Walworth last night.
A maisonette on the first, second and third floors of the block was damaged by fire.
Two women and two men left the property before the arrival of the Brigade. One man was treated at the scene by London Ambulance Service crews from smoke inhalation before being taken to hospital.
The Brigade was called at 2241 and the fire was under control at 2351. Fire crews from Old Kent Road, Peckham, Dockhead and New Cross fire stations attended the scene.
The cause of the fire is under investigation.