Two women and a man are in a critical condition after they were found unconscious in the street.
When officers arrived at the scene close to the Zetland pub in Marske High Street, Cleveland, at 12.35am on Friday they found the man had minor external injuries but the women had no visible sign of being hurt.
They were taken by ambulance to the James Cook University Hospital, Middlesbrough, where all three are critical, Cleveland Police said.
A cordon has been put in place and police remain at the scene.
A Cleveland Police spokeswoman said: "The investigation is in its very early stages and officers are trying to establish the circumstances surrounding the incident."
Police would like to hear from anyone with information, particularly drinkers who were in the Zetland pub on Thursday night.