OBJECTIVES: To design and evaluate a theory-based intervention to encourage the use of protective helmets in school-age cyclists. DESIGN: Two-by-three mixed design on 97 cyclists who did not initially use a helmet: Condition (intervention/control) x Time (pre-intervention/immediately post-intervention/5-month follow-up). METHOD: The intervention builds on a previous study using the Theory of Planned Behaviour in which we identified a small number of salient beliefs that predict intention to use a safety helmet and helmet use (Quine et al., 1998). Participants were randomly assigned to intervention or control conditions. The intervention group was presented with a booklet containing a series of persuasive messages based on the identified salient beliefs, and the control group was presented with a different series of messages concerning a cycling proficiency and bicycle maintenance course. Initial beliefs were measured just before the intervention at Time 1, by questionnaire. The immediate effects of the intervention were evaluated by questionnaire at Time 2. Five months later, at Time 3, the long-term effects of the intervention on beliefs, intentions, and behaviour were assessed. RESULTS: The behavioural, normative and control beliefs and intentions of intervention participants became more positive than those of control participants, and the effect was maintained over time. There was also a significant effect on behaviour: at 5-month follow-up, none of the 49 control children had taken up helmet wearing, while 12 (25%) of the 48 intervention children had. CONCLUSIONS: The results suggest that in order to promote lasting helmet use in young cyclists, we need to change their beliefs. The intervention reported here may present an inexpensive solution to the problem of persuading adolescents to use safety helmets. The results point to the value of social cognition theories such as the Theory of Planned Behaviour in the design of effective interventions to change health behaviours.