A 15-year-old girl suffered serious head injuries after falling from a moving car in Pinjarra during the early hours of Friday morning.
A white Holden Commodore was travelling along Padbury Road when the girl fell from the top of the boot area, hitting her head on the roadway.
The girl was rushed to Peel Health Campus by ambulance before being transferred to Royal Perth Hospital.
A 17-year-old female driver is currently assisting Major Crash Investigation officers with their inquiries.
Police are urging anyone who saw this incident, or the Holden Commodore being driven in the area prior to the crash, to call Crime Stoppers on 1800 333 000 or report the information online at crimestopperswa.com.au