Four people were injured, including one seriously, in a single-vehicle crash on Highway 9 in Caledon.
Ontario Provincial Police said it occurred on the highway between Horseshoe Hill Road and Third Line Sunday afternoon.
One person was taken to a trauma centre with life-altering injuries, and three others were transported to hospital with minor injuries, the OPP said.
The cause of the crash is unknown.
Highway 9 is expected to be closed for several hours while police investigate.