A joint police and fire investigation has begun into a huge blaze at the old Stobhill Hospital in Glasgow.
More than 100 firefighters were called to the fire in Balornock Road, Springburn, on Tuesday evening.
Detectives said the fire had the potential to affect the neighbouring working hospital on the same site.
Firefighters remained overnight at the scene of the blaze, which has now been brought under control. There are no reports of casualties.
A spokesman for NHS Greater Glasgow and Clyde said the fire broke out in the former out-patient and radiology departments and had completely destroyed the building.
A number of category B listed buildings are on the site, which opened to provide medical care for the poor in 1904.
New Stobhill Hospital
A replacement hospital specialising in areas such as day surgery opened in 2009, with in-patient and A&E services transferred to Glasgow Royal Infirmary.
The spokesman added that the neighbouring new Stobhill Hospital building had been unaffected by the blaze and that appointments would be held as normal.
The fire service said the fire was now limited to a "few pockets" and that the flames were still being extinguished.
Gordon Pryde from the Scottish Fire and Rescue Service described the blaze as a "challenging and protracted incident".
'Protect surrounding buildings'
He added: "A phenomenal amount of work has been done overnight to bring this fire under control, and to protect surrounding buildings.
"I must praise the response and professionalism of our firefighters - they have been exceptional."
Police Scotland has appealed for anyone with information about the fire to come forward.
Det Insp Craig McPhail added: "Any fire is dangerous, and the incident last night at the derelict building at Stobhill Hospital had the potential to affect the working hospital on the same grounds.
"Thankfully nobody was inside the derelict building and no neighbouring buildings needed to be evacuated.
"The Scottish Fire and Rescue managed to get the fire under control quickly and it has now been extinguished. We will now work alongside them to establish exactly what has happened."