The incident happened around 5:30 p.m. Monday night along Allegheny Springs Road in Warren County. Two teens were riding their ATV's back-to-back along the road when the driver in front braked suddenly to make a left turn onto a trail.
The ATV behind him braked but was unable to avoid slamming into him. Both of the teens were thrown from their vehicles.
One of the teens was flown by Life Star from Warren General Hospital to Hamot Medical Center for serious injuries. The other teen was not hurt.
State Police say the teens were not wearing helmets.