Intervention study, investigating the effect of use of a client-accessible electronic patient record for preventive child health and youth care (EPR-Youth) on experienced client autonomy and the role of professional autonomy-supportive behaviour in strengthening this effect. To evaluate effect of EPR-Use on client autonomy and the role of professional attitude in strengthening that autonomy, A mixed methods design combined baseline and follow-up questionnaires with focus group interviews, over a period of two years. Different samples of clients completed a questionnaire about experienced autonomy and portal use at start (1202 parents, 202 adolescents) and 12 months after implementation (914 parents, 89 adolescents). Professionals completed questionnaires about autonomy-supportive behaviour at baseline (N=100), after 5 months (N=57) and 24 months (N=110). After 14 months, focus group interviews were held with separate groups of clients (eight parents, four adolescents) and professionals (N=12).