The road has now been re-opened.
Police confirmed that a woman suffered serious head injuries and has been airlifted to Derriford Hospital by an RNAS Culdrose Sea King helicopter.
The incident involved two vans and Peugeot car.
Police are appealing for witnesses to the collision and anyone with information is asked to contact them on 101.
A woman is understood to be trapped in a car after a multi-vehicle crash on the A39 between Truro and Falmouth.
Emergency services are on the scene of the crash about 200 metres south of the Norway Inn on the busy commuter stretch.
The incident is thought to involve two vans and a car and to be near the Perran Foundry housing development site.
The A39 is closed in both directions between the Treluswell roundabout coming out of Penryn and the Devoran roundabout.
Diversions are in place and traffic is reported to be very heavy in the Ponsanooth and Perranwell areas as a result.
Several crews from Cornwall Fire and Rescue service are on the scene and are thought to be working to get the woman out of the car.