Background: Numerous studies have looked at the efficacy of Parent-Child Interaction Therapy (PCIT) for young children with externalizing behaviour problems. Objective: The present study compiled these results through a comprehensive review to provide greater clarity regarding the efficacy of this treatment. Methods: Using a random effects model, a meta-analysis was conducted to determine the weighted mean effect size. To be included in this analysis, studies were required to have implemented PCIT with children (ages 2-5) with clinically significant externalizing behaviour problems. Twelve studies comprising 254 treated and 118 control group children were included, with the majority of children being White males. This research also assessed whether gender and type of disruptive behaviour disorder (DBD) moderated the effectiveness of PCIT. Results: PCIT had a large effect on improving externalizing behaviour problems in children with DBD based on the effect size derived from pre- and post-treatment behavioural outcomes (d = 1.65, 95 % CI [1.41, 1.90], p < 0.001) and treatment and control group data (d = 1.39, 95 % CI [1.05, 1.73], p < 0.001). Neither gender nor diagnosis was found to significantly moderate the effectiveness. Conclusions: PCIT was found to be an efficacious intervention for child DBD, although the small number of eligible studies and lack of diversity in the sample populations suggests a need for further research. This study has important implications for both practitioners and researchers and provides an efficient summary of the research to date.