This paper examines the effectiveness of a group-based universal parent training program as a strategy to improve parenting practices and prevent child problem behavior. In a dissemination trial, 56 schools were first selected through a stratified sampling procedure, and then randomly allocated to treatment conditions. 819 parents of year 1 primary school children in 28 schools were offered Triple P. 856 families in 28 schools were allocated to the control condition. Teacher, primary caregiver and child self-report data were collected at baseline, post, and two follow-up assessments. Analyses were constrained to highly adherent parents who completed all four units of the parenting program. A propensity score matching approach was used to compare parents fully exposed to the intervention with parents in the control condition, who were matched on 54 baseline characteristics. Results suggest that the intervention had no consistent effects on either five dimensions of parenting practices or five dimensions of child problem behavior, assessed by three different informants. These findings diverge from findings reported by program developers and distributors. Potential explanations for the discrepancy and implications for future research are discussed.