Rescuers responded to the scene, at the intersection of I-90 and Highlands Drive NE, and found the motorcyclist lying in the creek bed with multiple serious injuries.
He was removed from the creek, treated by Eastside Fire medics at the scene and then taken to Harborview Medical Center.
The State Patrol said a trooper noted both a blue and yellow motorcycle speeding along I-90, but when he turned on his lights, the motorcycles sped off even faster.
The trooper said he estimated the blue motorcycle was going between 100 and 140 mph and backed off the chase. Meanwhile, the yellow motorcycle passed the trooper and then exited to Highlands Drive. The trooper slowed and turned off his lights, but moments later that motorcycle lost control on the offramp's turn and launched off the East Sunset Way overpass into Issaquah Creek, 68 feet below.
The rider on the blue motorcycle was able to get away and is still at large. Anyone with information that may lead to the whereabouts of the second motorcycle is asked to call the State Patrol at 425-401-7788.