Fire caused up to $300,000 in damage Friday to a wing of the old Healdsburg District Hospital, a boarded-up facility that is sometimes occupied by homeless people, fire officials said.
Black smoke and flames were reported coming from the Johnson Street building at about 6:55 a.m., fire Chief Steve Adams said.
Firefighters arrived to find the blaze near a central kitchen area, Adams said.
It was contained in about a half-hour, he said.
"The interior sustained heavy damage," Adams said. "Once the hallway skylight burned, all the heat and smoke went out, allowing our crews to get inside."
He estimated the damage at between $200,000 and $300,000.
The cause was under investigation but he said it appeared to have been sparked by someone in the building, Adams said.
He said transients were recently seen inside and the city has been in continued talks with the property owner about securing the building.