Background: Mental problems in children are widespread and cannot be reduced through treatment only. Prevention is therefore urgently needed although it is unclear how effective such strategies may be, particularly in the long term. Aim: Can a parent-centered universal prevention program that is effective in the short term also yield effects after 10 years? Method: According to their preschool location, N = 477 families were randomly assigned to the parent training prevention program (Triple P Positive Parenting Program, TP; Sanders, 2012) or the control group (CG). In all, 77% accepted the TP offer (T+), while 23% declined it (T‐). The 10-year effectiveness of the program was established with self-report measures of mothers, fathers, and adolescents from N = 361 families. Results: The intention-to treat analysis (comparison TP vs. CG) yielded negligible findings. By contrast, the differential analysis from the T+ mothers’ perspective found long-term improvements in Child Behavior Checklist (CBCL) internalizing and externalizing behavior and relationship satisfaction in comparison with CG and T- mothers. At 10 years, compared with pre-assessment, T+ mothers reported the smallest increase in the CBCL sum score of internalizing and externalizing behaviors, 5%, while CG (20%) and T- (33%) mothers reported far higher rates. Contrary to the hypotheses, parenting behavior did not change over time. T+ fathers reported improvements in parenting behavior, while adolescents reported negligible outcomes. Conclusion: The results support the long-term effectiveness of the TP program as a universal prevention intervention, at least from the T+ mothers’ perspective. More research should be conducted with the T- families because they showed worse outcomes than the control group
