The aim of this prospective study was to quantify the anatomic severity of head and cervical spine injuries in hospital admitted victims of motorcycle and moped accidents in relation to helmet use and controlled for non-head injuries (i.e. kinetic impact). Two hundred and twenty-three patients entered the study group, of which 152 were motorcyclists and 71 were moped riders. Our results reveal that helmets do prevent head injury in motorcycle and moped accidents, especially in those crashes involving relatively low kinetic energy transfers. Helmet use does not lead to an increase of the incidence or severity of cervical spine injury. As a result compulsory helmet laws should not be limited to motorcyclists but also focus on all moped riders and probably also bicyclists. This study illustrates that emergency departments can provide important epidemiological information for injury control purposes. However, the epidemiological use of emergency department data and hospital data in general requires cautiousness. Confounding is a common problem which should be dealt with during analysis.