Behavioural parent training programs have been developed to address child behaviour problems through improvement in parenting practices. Ideally, programs would demonstrate effectiveness with all parents. The Triple P-Positive Parenting Program is widely reported as an effective, evidence-based program for parents. However, in this meta-analysis we demonstrate that there are significant differences in program effectiveness for mothers and fathers. We show that while Triple P has a large positive effect on mothers' parenting practices, it has a smaller effect on fathers' parenting practices. Considering that fathers make a significant and unique contribution to child development, we argue that it is important for parenting programs to assess their effectiveness with fathers as well as mothers.