Police say the bikers were behind a car when traffic slowed in the northbound lanes near Wilsonville, south of Portland.
They say the bikers and car tried to slow but collided, scattering bikes across the pavement.
The Oregonian in Portland reports that two bikers with critical injuries were flown to Portland hospitals by helicopter. Rescue personnel say seven others were treated for shoulder and hip injuries and broken bones.
The freeway reopened about 6 p.m., nearly four hours after the accident.
Information from: The Oregonian, http://www.oregonlive.com