The blaze erupted near the ICU at 1111 West La Palma Avenue around 10:45 p.m.
The sprinkler system in the hospital was triggered, putting out the flames.
The hospital was able to evacuate the area before firefighters and police arrived.
No one was hurt.
The scene was cleared around 12:19 a.m. and hospital operations resumed.
No information on what kind of damage was done to the building has been released.