The A9 was shut in both directions after the collision south of Dalwhinnie between a lorry and a car at around 00.10am today.
Police Scotland said that casualties were removed from the scene by helicopter and ambulance.
There is no official diversion and an alternate route takes four hours and involves a 152 mile drive.
Police said: "Motorists are asked to be patient whilst emergency services carry out investigations into the cause of the collision."
Traffic Scotland advised motorists to allow extra time for their journeys.
The Traffic Scotland website said: "No official diversion route has been established but an alternate route is available for northbound traffic from Perth via the A85-A82-A86.
"Southbound traffic can use the reverse of this route but either direction consists of a four hour drive and approximately 152 miles."
It said the road was closed between Wade Bridge and Dalnacardoch Lodge half a mile south of the Dalwhinnie Junction with the A889.
The Scottish Ambulance Service said that two people were taken by helicopter to Ninewells Hospital in Dundee while one was taken there by road ambulance.
Two people were taken to Raigmore Hospital in Inverness by road ambulance.
The road reopened just before 9am.
The five people injured were all men who were in the car.
Police said that one man is in a life-threatening condition, one is in a serious condition and the other three were not seriously injured.
The man driving the lorry was not injured.