Problem: Recent reports about bicycle helmet wearing indicate that the number of helmet users is still very small among teenagers. The objective of this prevalence survey was to investigate why teenagers do not use a bicycle helmet even if they have one. Method: Data were collected at two schools in Helsinki, Finland. High school students (N = 965) completed a questionnaire about their cycling habits and bicycle helmet use. Results: A student's parents' positive attitude to bicycle helmet use was the strongest predictor of having a helmet. Analyses of responses given by bicycle helmet owners showed that having friends who use a bicycle helmet is strongly related to a student's decision to wear a helmet. In addition, parents' positive opinion to helmet wearing predicted helmet use frequency. Other factors accounted only for a small proportion of variance in helmet wearing frequency. Summary and Impact on Industry: The present study shows that the most efficient way of increasing bicycle helmet-wearing rate among students is to influence peer opinions and to inform students' parents about the safety benefits of bicycle helmets, which should be taken into account when planning bicycle helmet-wearing campaigns and other countermeasures. (C) 2001 National Safety Council and Elsevier Science Ltd. All rights reserved.