Head injury is the most common cause of morbidity and mortality in motor vehicle crashes. Efforts to improve vehicle design, which minimize forces exerted to the occupant's head, may lead to a reduction in the frequency and severity of head injury. We therefore set out to identify mechanisms producing severe head injury in motor vehicle crashes (MVC) derived from the crash injury research and engineering network (CIREN) database. CIREN combines crash site analysis, vehicle damage assessment, and occupant kinematics in relation to the occupant's injuries. From the Seattle CIREN database of 101 cases, compiled from 1997 to 1998, we selected those crashes in which the occupant sustained severe head injury (abbreviated injury score, AIS>or=4) for analysis. We examined crash mechanism, energy transfer, point of head contact, vehicle intrusion and resulting injuries. There were 15 cases with severe head injury. These were primarily due to side impacts (n=10) in comparison to front impacts (n=5). The average net change in velocity (delta velocity, DV) was 15 mile/h (range 4-29 mile/h). In cases where the primary point of head contact could be elucidated the B-pillar predominated (4 cases, 33.3%) followed by the striking external object (2 cases, 16.7%), A- (1 case, 8.3%) and C- (1 case, 8.3%) pillars, roof side rail (1 case, 8.3%), windshield header (1 case, 8.3%), windowsill (1 case, 8.3%) and airbag (1 case, 8.3%). In this series the predominant mechanism of head injury was lateral impacts, especially those in which the victims' heads struck the B-pillar. The need for improved head protection from lateral impacts is indicated.