Four fire engines and around 25 firefighters were called to a fire at a derelict doctors' surgery on Moorfield Road in Enfield.
Part of the first floor of the two-storey building was damaged by the fire. There were no reports of any injuries.
Station Officer Tom Robinson, who was at the scene, said: "There was a lot of thick black smoke coming from the first floor window of the building when firefighters arrived.
"It was a challenging fire for crews to gain access to as the building was boarded up but they worked hard to gain entry and tackle the fire and were able to limit the damage."
The Brigade was called at 0742 and the fire was under control 0822. Fire crews from Enfield, Edmonton, Chingford and Tottenham fire stations were at the scene.
The cause of the fire is under investigation.