Maria de Jesus Quiej Alvarez and Maria de Teresa Quiej Alvarez were gently carried from a private jet at Burbank Airport, placed in an ambulance and taken to Mattel Children's Hospital at the University of California-Los Angeles.
Doctors were expected to order tests for the girls and review the findings, said a spokeswoman for UCLA Medical Center. The conditions of the 2-year-old twins were not released.
The girls' parents did not travel with them from Guatemala, and it was unclear if they would come on a later flight.
The girls had been scheduled to visit the United States in coming weeks for a routine checkup, but the trip was moved up after both experienced health problems.
Maria de Jesus was rushed to a hospital Wednesday with a fever and convulsions, and her sister may have to undergo surgery in the United States, Dr. Ludwig Ovalle said.
Maria de Teresa is receiving oxygen to help her breathe while recovering from a May 2 operation to replace a valve, which relieves pressure on the girl's brain by allowing accumulated liquid to escape.
Doctors said both girls are well enough to travel.
Born July 25, 2001, in Mazatenango, 110 miles southeast of Guatemala City, the twins drew the attention of Guatemalan and U.S. medical charities.
The Guatemalan Pediatric Foundation and other humanitarian groups raised the money to have the two surgically separated Aug. 6 at the Mattel Children's Hospital of the University of California, Los Angeles. The operation lasted 23 hours and made headlines around the world.
The twins returned to Guatemala in January, and their parents were given a new house in the capital so they could be near their daughters' doctors.
It was unclear how long they will be in the United States on this trip.
By Sandra Marquez