Acceptance and commitment therapy (ACT) is a contextual behavior therapy based on relational frame theory (RFT), a functional-contextual approach to human language and cognition. The main aim of ACT is to promote psychological flexibility, which has been defined in middle-level terms. The current study proposes that a recently developed RFT account of psychological flexibility might facilitate the adaptation of ACT to the work with young children.A case study with a 5-year-old boy presenting problematic anger is presented to illustrate the previous suggestion. The intervention consisted of a brief ACT protocol applied in four, 20-min sessions and a token economy at the service of promoting psychological flexibility. The ACT protocol involved a multiple-exemplar training in framing own ongoing experiences through deictic and hierarchical relations and transforming the discriminative functions of those experiences by relating them to rules that progressively specified longer term and symbolic, positive consequences.The intervention was shown to be highly effective in reducing the anger episodes, and the results were maintained during the one year of follow-up.This study supports the feasibility of the RFT account of psychological flexibility to adapt ACT to the work with young children.