Four fire engines and around 25 firefighters tackled a fire at a hospital wing on Blackshaw Road in Tooting.
Part of a generator room on the lower ground floor of the building was damaged by fire. There were no reports of any injuries.
The Brigade was called at 1624 and the fire was under control by 1718. Fire crews from Tooting, Brixton, Mitcham and Wimbledon fire stations attended the scene.
The fire is believed to have been accidental and caused by heat coming from the generator.