The woman was a passenger in a black Vauxhall Astra heading south on the A82 near Loch Lomond on Friday afternoon.
The car collided with a black Mercedes Vito mini-bus travelling in the other direction at about 2.50pm.
The 99-year-old woman died at the scene of the crash, just south of Luss.
The 73-year-old Astra driver was taken by ambulance to the Royal Alexandra Hospital in Paisley, where hospital staff said she was in a stable condition.
The driver and five passengers on the bus sustained minor injuries and did not have to go to hospital.
Sergeant Kevin Craig, of the road policing department in Dumbarton, has appealed for witnesses and anyone with information to contact him on 101, quoting incident number 2348 of March 29.