A quad biker and a cyclist collided in a serious head-on smash on the Spen Valley Greenway.
Two men were injured after the collision on the greenway, off Carr Lane, Dewsbury Moor, at 7pm on Sunday.
Police said the cyclist was heading towards Ravensthorpe and the quad rider towards Heckmondwike.
A Yorkshire Air Ambulance spokeswoman said one of the men was airlifted to Leeds General Infirmary.
Any witnesses are asked to contact the Southern Area Roads Policing Unit on 01924 293055.