The aim of the present study was to test the effectiveness of the Prevention Program for Externalizing Problem Behaviour (PEP). PEP has already been evaluated as an efficacy study, i.e., with high internal validity. The main concern of the current study was to test the effects of PEP under conditions of high external validity.58 child therapists of 37 local institutions of the public health system held PEP trainings. The intervention was evaluated by a within-subject control group design with three assessment points, two before (pre1 and pre2) and one immediately after (post) the PEP training. Depending on the type of analysis, 198 up to 277 families with children aged 3-10 years were included. Data were collected by questionnaires and comprised symptomatology of the child and the mother as well as parenting. For parenting and child behaviour problems, changes after treatment were greater than changes during the waiting period. PEP produced effect sizes in the small to moderate range. The results suggest that PEP can be delivered under routine care settings without loss of positive treatment effects. As worse results are often obtained in effectiveness trials, these findings have to be regarded as particularly promising.