A three-vehicle crash closed all lanes of Highway 400 northbound in Vaughan into the Tuesday morning rush hour.
The collision happened around 10:30 p.m. Monday north of Teston Road.
A crossover vehicle hit an SUV, causing the SUV to rear-end a tractor trailer, according to Ontario Provincial Police.
The driver and a passenger in the SUV were taken to hospital with life-threatening injuries, police said.
The extensive clean-up required means the northbound lanes will likely remain closed until about 8 a.m.
The Aurora OPP detachment is investigating the crash. Anyone with information or dashcam video is asked to contact police.