The crash happened around 11 a.m. on University Avenue near Chaucer Street as a Bay Medic ambulance was transporting the 56-year-old woman from Concord to a Palo Alto medical facility for a routine procedure, police said.
The ambulance was traveling west on University Avenue when, for an unknown reason, it left the roadway, hit a tree and rolled over, according to police.
The woman suffered major injuries and was taken to Stanford Hospital. She was listed in "very grave condition" today, Ryan said.
Three ambulance personnel suffered minor injuries and were also transported to Stanford Hospital.
Ryan said police are still investigating the cause of the crash, but that road conditions may have played a role.
"It was raining, the streets were wet and there were piles of leaves that were wet, too," Ryan said. "They all could have been a factor."