Objective To determine the risk factors for serious injury to bicyclists, aside from helmet use.

Design Prospective case-control study.

Setting Seven Seattle area hospital emergency departments and two county medical examiner's offices.

Patients Individuals treated in the emergency department or dying from bicycle related injuries.

Measurements Information collected from injured bicyclists or their parents by questionnaire on circumstances of the crash; abstract of medical records for injury data. Serious injury defined as an injury severity score>8.

Analysis Odd ratios computed using the maximum likelihood method, and adjusted using unconditional logistic regression.

Results There were 3854 injured cyclists in the three year period; 3390 (88%) completed questionnaires were returned. 51% wore helmets at the time of crash. Only 22.3% of patients had head injuries and 34% had facial injuries. Risk of serious injury was increased by collision with a motor vehicle (odds ratio (OR)=4.6), self reported speed >15 mph (OR=1.2), young age (<6 years), and age >39 years (OR=2.1 and 2.2 respectively, compared with adults 20-39 years). Risk for serious injury was not affected by helmet use (OR=0.9). Risk of neck injury was increased in those struck by motor vehicles (OR=4.0), hospitalized for any injury (OR=2.0), and those who died (OR=15.1), but neck injury was not affected by helmet use.

Conclusions Prevention of serious bicycle injuries cannot be accomplished through helmet use alone, and may require separation of cyclists from motor vehicles, and delaying cycling until children are developmentally ready.