Firefighters were called to a house fire on Fleetwood Road in Dollis Hill.
Part of the ground-floor living room of a semi-detached house was damaged by fire. Five adults and two children left the building before the Brigade arrived. A man, a woman and two children were treated on scene by London Ambulance Service crews after suffering from smoke inhalation.
The Brigade was called at 0521 and the fire was under control by 0618. Two fire engines from Willesden and West Hampstead fire stations attended the scene.
The cause of the fire is under investigation.