Ten fire engines and around 70 firefighters tackled a fire on Brockley Hill in Stanmore.
Most of a disused two-storey hospital building, including its roof, was damaged by fire. There have been no reports of any injuries at this time.
The fire has been brought under control by firefighters but crews have remained on site throughout the evening working to extinguish the fire completely.
A 32-metre turntable ladder has been at the scene and the Brigade's drone team have also been in attendance to assist crews.
Road closures have been in place in the area while the incident has been ongoing.
The Brigade was called at 1705 and the fire was brought under control by 1911. Crews from Stanmore, Mill Hill, Kentish Town, Finchley, Paddington and surrounding fire stations attended, along with Hertfordshire Fire & Rescue Service.
The cause of the fire is under investigation.