Drugs or alcohol were believed to be a factor in the crash.
State troopers responded to the scene, near the intersection of Highway 9 and 136th Street SE, at about 11:15 p.m. Wednesday after receiving a report of a serious accident.
A preliminary investigation found that a 2010 Harley Davidson Fatboy with a man and woman aboard was heading south on Highway 9 when it crossed the center line into the northbound lane.
The motorcycle crashed head-on into a 2011 Nissan Versa driven by a 59-year-old Tulalip man.
The motorcycle's driver, a 44-year-old Snohomish man, and the passenger, a 31-year-old Snohomish woman, were taken to Harborview Medical Center for treatment of serious injuries. Both were wearing helmets.
The driver of the car was uninjured.
The accident remains under investigation and possible charges are pending.