To improve regular hand hygiene in adolescents, educational messages based on medical information have not been very successful. Therefore, a theory-guided self-regulatory intervention has been designed with a particular focus on planning strategies. A randomised controlled trial with 307 adolescents, aged 12-18 years, was conducted in high schools. The control group received educational hand hygiene leaflets, whereas the experimental group received a self-regulatory treatment which required them to generate specific action plans and coping plans. Three times during one month, both groups received verbal reminder messages about planning to wash their hands properly. At one-month follow-up, hand hygiene behaviour as well as planning to practise hand hygiene were higher in the self-regulation than in the education group (p < .01). Moreover, changes in planning levels operated as a mediator between experimental conditions and changes in behavioural outcomes. Teaching self-regulatory planning strategies may constitute a superior approach than educational messages to improve regular hand hygiene practice in adolescents.