Résumé : The assessment of externalizing behavior (EB) in preschoolers is crucial in developmental psychopathol-ogy. In the absence of any gold standard measure, new tools contribute to a multi-method and multi-informantapproach. The aim of the current study is to present and validate a new observational paradigm, the UnfairCard Game (UCG), intentionally structured to increase the likelihood that negative affect, agitation andinattention will emerge during a video-recorded task. It was administered to 268 young children, and theresults were validated by means of factorial analysis, reliability analyses, inter-rater agreement, test-retest,discriminant analyses and external validation with the Child Behavior Checklist (Achenbach Rescorla,2004). The validity of the UCG is supported by the findings.