Southwest flight No. 508 left Oakland for Burbank at 7:12 a.m.
A short time later, the captain radioed back saying he was returning to Oakland because a flight attendant had begun to feel lightheaded and complained of an unusual smell in the cabin, airport spokeswoman Joanne Holloway said.
"The captain elected to return to Oakland," Holloway said.
The plane landed at the Oakland airport at 7:27 a.m. and the flight attendant was checked out, then taken to a hospital for further evaluation, Holloway said.
It does not appear that anyone else suffered symptoms, she said.
Southwest Airlines spokesman Brad Hawkins said there were 137 passengers on the plane, plus a crew of five. He said the passengers are being put onto another flight and will probably arrive in Burbank about two hours later than scheduled.
The source of the odor has not yet been determined.