A boy is in a critical condition after he was thrown from a car which landed on top of him.
Police are appealing for witnesses following the incident, which happened in Belmont, near Chorley, at around 11.10pm on Thursday.
Officers said a black Citroen C2 was travelling along a series of bends in Rivington Road when it left the road and ended up on a grass bank, rolling several times.
A 16-year-old boy, from Bolton, was thrown from the vehicle, which then landed on top of him causing him serious injuries.
A 19-year-old man, also from Bolton, flagged down a passing car and the occupants were able to lift the C2 from on top of the boy, before first aid was carried out by a nurse who was also passing the scene.
Both teenagers were taken to Royal Preston Hospital. The 19-year-old suffered a chest injury and minor cuts, while the 16-year-old is being treated for a serious head injury as well as internal injuries.
Sgt Jon Allwright said: “An investigation into the circumstances surrounding this collision is now underway.
“Although a number of people stopped at the scene, there is the possibility that there are other people who may have seen the Citroen being driven elsewhere during the evening.
“If you think you may have seen the car and have any information which you believe could help then please come forward.”
Anybody with any information can contact police on 101 or Crimestoppers, anonymously, on 0800 555 111 or online at www.crimestoppers-uk.org.