Thick fog delayed the time in the alarm being raised about the night-time blaze at Dunain House in Inverness.
The building, constructed in the mid-18th century, had been part of Craig Dunain Hospital in Leachkin.
Firefighters remain at the scene, but say the fire that has destroyed the building which was part of the former Craig Dunain Hospital.
The fire at Dunain House, which is just off the A82 on the outskirts of Inverness, was reported at 10.30pm last night.
Six fire appliances - three from Inverness, one from Beauly, one from Drumnadrochit and one from Dingwall - were sent to the scene. An aerial platform and a water carrier from Inverness were also involved as was the welfare unit and command unit.
Incident Commander, Group Manager John MacDonald, was at the scene and said this morning: “On arrival the crews were faced with a well-developed fire. It appeared to have been burning for some time prior to our arrival, but that was probably due to the fact it was difficult to see due to thick fog last night.
“The fire was split into two operational sectors and due to the instability of the building we fought the fire from the outside. We used the aerial platform and its large water monitor to attack the fire from above and used four main jets on the ground.
“The fire was brought under control by around 4am. The building suffered a complete collapse of the roof and internal walls.”
He added: “Crews remain at the scene and due to the unstable nature of the building we are carrying out damping down operations externally. We are likely to be at the scene for most of the day today.
“I would like to pay tribute to the attending crews who worked tirelessly through the night to bring the incident under control and ultimately to a safe conclusion.”
A joint investigation with Police Scotland will be carried out in due course into the cause of the fire.