Thames Valley Police is appealing for witnesses following a serious injury collision near Bow Brickhill.
Emergency services were called to Brickhill Road at around 4:45am on Saturday.
The single vehicle collision occurred when a grey VW Golf travelling along Brickhill Road, towards Bow Brickhill, veered across the carriageway and crashed into a ditch.
The driver, a 25-year-old woman sustained serious injuries. The three passengers, two men and one woman sustained minor injuries. All four people were taken to Milton Keynes hospital.
Investigating officer PC Phil McGlue, from the Joint Operations Unit for Roads Policing, said: “I would like to speak to anyone who witnessed the collision or who has any information which could assist our investigation."
Anyone with information is asked to call 101.
