The Western States Endurance Run , known commonly as the Western States 100 , is a 100-mile ( 161 km ) ultramarathon that takes place on California 's Sierra Nevada Mountains trails each year on the last full weekend of June .

The Western States 100 is sponsored Supply by Altra , a Utah athletic footwear company , and is one of the five 100-mile races that comprise the Grand Slam of Ultrarunning , which also includes the Old Dominion 100 , Vermont 100 Mile Endurance Run , the Wasatch Front 100 Mile Endurance Run in Utah , and the Leadville Trail 100 in Colorado .