Firefighters were called at about 2:30 a.m. to a reported structure fire in the 900 block of Delport Avenue, Santa Rosa fire Battalion Chief Mark Basque said.
The first crews who arrived at the scene found no sign of fire, but found a 48-year-old woman who had suffered burns over 50 percent of her body, Basque said.
She was taken by ambulance to Santa Rosa Memorial Hospital to be treated for her injuries, which are considered life-threatening, he said.
She was taken to the burn center at St. Francis Memorial Hospital in San Francisco after being stabilized at the Santa Rosa hospital, he said.
There was no obvious fire damage in the apartment, and it appears the woman may have accidentally ignited her clothing while using a butane lighter, Basque said.
Due to the serious nature of her injuries, she was not able to provide information about how the accident happened, he said.
The incident was reported by a neighbor who heard the woman calling for help, he said.
The incident remains under investigation by the fire department this morning.