Background Approximately 20% of children experience internalizing or externalizing DSM-IV-TR disorders. This prevalence rate cannot be reduced through treatment only. Effective preventive interventions are therefore urgently needed. The aim of the current investigation is to evaluate the two-year efficacy of the group Triple P parenting program administered universally for the prevention of child behavior problems. Methods Based on their respective preschool, N = 280 families were randomly assigned either to the parent training or to the control group. The efficacy was analyzed using multi-source assessments, including questionnaires by mother and father, behavioral observation of mother-child interaction, and teacher evaluations. Results At the 2-year follow-up, both parents in the Triple P intervention reported significant reductions in dysfunctional parenting behavior, and mothers also an increase in positive parenting behavior. In addition, mothers reported significant reductions in internalizing and externalizing child behavior. Single-parent mothers in the Triple P intervention did not report significant changes in parenting or child problem behavior which is primarily due to inexplicable high positive effects in single parent mothers of the control group. Neither mother-child interactions nor teacher ratings yielded significant results. Conclusions The results support the long-term efficacy of the Triple P - group program as a universal prevention intervention for changing parenting behavior in two-parent households, but not necessarily in single-parent mothers.