Seven-year-old twins have been flown to hospital following a serious crash involving four cars.
The collision happened on the A174 near Yearby, outside Redcar, at 10.15am on Boxing Day, Cleveland Police said.
A boy and a girl were flown to the James Cook University Hospital, only five minutes away from the scene, in two helicopters. An emergency services source said the children were twins.
The boy was seriously ill having suffered head and leg injuries. The condition of his sister was not clear.
One of the cars involved ended up on its roof.
Cleveland Police said a "number of other casualties" were taken by paramedics to the same hospital.
The A174 was closed in both directions and was expected to remain shut for a number of hours.
Any witnesses should call the Cleveland and Durham Specialist Operations Unit on 101, quoting reference number 230130.
The Great North Air Ambulance Service said it treated the boy at the scene before flying him to hospital.
A spokesman said the girl was flown to the same hospital in the police helicopter. More information about the casualties was expected to be released later.