This research set out to recognize and overcome the difficulties and doubts in designing, testing and disseminating a dental health education programme for adolescents, as part of their school curriculum. After extensive pilot work, large-scale field trials were mounted of Natural Nashers for 13-14-year-old pupils in school. These trials ran among 6700 pupils in 45 largely urban, mixed sex, state comprehensive schools in England and Scotland. A quasi-experimental control group design was used and evaluation was carried out at the levels of process (teachers' questionnaires, pupils worksheets, field-workers' reports) and outcome (pupils' pre- and post-questionnaires, worksheets, plaque and gingival scores), among a random subsample (n = 1900) divided between study (n = 1300) and control (n = 600). Eighty-three teachers completed a questionnaire and 330 field workers' reports were analysed. Two hundred and fifty pupils were further examined after a minimum of 6 months. Significant improvements in plaque and gingival scores, knowledge and attitudes compared to control pupils were noted. Gains in the areas of knowledge and attitudes were the largest of any recorded in similar trials in Europe. Process data showed that in the experimental schools, improved oral hygiene was correlated with teachers who were enthusiastic and used a cooperative style. Results from the teachers' questionnaire showed that the programme was very well received and educationally suitable.