Three children are also in hospital with serious injuries.
Police say the collision happened just after midnight at Highway 56 and Golf Club Road.
The drivers of both cars – a 42 year old man from Glanbrook, Ont., and a 17-year-old boy from Stoney Creek – died in the crash.
Two daughters of the older man and one of their friends were taken to hospital. They are expected to survive.
Police do not know what caused the collision.
A police investigation team is on site and Highway 56 is closed between Golf Club Road and Guyatt Road.