Locals have called for action after crews were called to a derelict hospital on the outskirts of Dundee on Friday.
Firefighters wearing breathing apparatus used a hosereel to put out rubbish on fire in the grounds of Strathmartine Hospital.
Three appliances attended the incident after the alarm was raised at 2.30pm.
No one was hurt in the incident.
There have been a series of fires at the former hospital in recent months while the main building was gutted by a major blaze in April last year.
Developers wants to spend £50 million turning the derelict hospital into luxury flats.
Karen McAulay from Strathmartine Hospital Histories said she feared some listed buildings may be badly damaged by the fires.
She said: “A lot of the buildings are probably structurally sound enough to take the consequences of a fire but some of the listed buildings are a concern.”
She added: “There was still quite open access to the site last month so I don’t think they’re going to get it under control.
“Something has to happen – whether that is putting somebody up there full-time as security or something else but, at the end of the day, if you buy a site like that you have to make sure you have the money for security.
“But I think this is just going to be a recurring problem.”
Last month Angus councillors were told that firebugs from Dundee may be responsible for the spate of blazes at the hospital.
Scottish Fire and Rescue Service said there had been 15 incidents in the Monifieth and Sidlaw ward, which includes the hospital, between April and June.
Fire chiefs are working with local community leaders and speaking to children in Dundee schools in an effort to cut the number of deliberately set blazes.
The increase was reported as part of the quarterly performance update for Angus, delivered to members of the local authority’s scrutiny and audit committee.
SFRS group manager Roy Dunsire told councillors that intelligence from Police Scotland suggested youths from the North East and Strathmartine wards of Dundee were starting the fires.
He said Strathmartine Hospital site, Burnside of Duntrune and Emmock Road were being frequently targeted.