PurposeAdolescents with congenital heart disease transition from childhood to adulthood and transfer from pediatric-oriented to adult-oriented care. High-level empirical evidence on the effectiveness of transitional care is scarce. This study investigated the empowering effect (primary outcome) of a structured person-centered transition program for adolescents with congenital heart disease and studied its effectiveness on transition readiness, patient-reported health, quality of life, health behaviors, disease-related knowledge, and parental outcomes e.g., parental uncertainty, readiness for transition as perceived by the parents (secondary outcomes). MethodsThe STEPSTONES-trial comprised a hybrid experimental design whereby a randomized controlled trial was embedded in a longitudinal observational study. The trial was conducted in seven centers in Sweden. Two centers were allocated to the randomized controlled trial-arm, randomizing participants to intervention or control group. The other five centers were intervention-naïve centers and served as contamination check control group. Outcomes were measured at the age of 16 years (baseline), 17 years, and 18.5 years. ResultsThe change in empowerment from 16 years to 18.5 years differed significantly between the intervention group and control group (mean difference = 3.44; 95% confidence interval = 0.27–6.65; p = .036) in favor of intervention group. For the secondary outcomes, significant differences in change over time were found in parental involvement (p = .008), disease-related knowledge (p = .0002), and satisfaction with physical appearance (p = .039). No differences in primary or secondary outcomes were detected between the control group and contamination check control group, indicating that there was no contamination in the control group. DiscussionThe STEPSTONES transition program was effective in increasing patient empowerment, reducing parental involvement, improving satisfaction with physical appearance, and increasing disease-related knowledge.