Snohomish County Fire District 1 says 13 patients were transferred to other hospitals during the incident, including one woman going into labor.
Fire District spokeswoman Leslie Hynes says no one was injured and the fire was controlled in 30 minutes.
Because of the possible danger, the fire went to three alarms and firefighters from three districts responded.
There were about 120 patients in the hospital at the time, but most were able to remain in their rooms.
The fire also knocked out power to parts of the hospital, but it was able to continue to operate on emergency generators.
Employee Kevin Brunink was inside the building when the fire started.
"The lights just went out, we lost the computers and the generator kicked in about 30 to 40 seconds later. They just started calling fire alarms on the 8th floor, 9th floor, 5th floor," he said.
Fifteen-year old Lacey Tolbert was trapped in a room on the fifth floor with her grandmother and her 12-year old brother when thick, grey smoke began wafting into the room.
"It was thick and it made your eyes burn and your lungs burn, too," Tolbert said.