The objectives of this paper are two-fold: to examine first, if the change from positive to negative alcohol and any other substance use status from baseline assessment to the onset of the first session (i.e., pre-treatment phase) occurs in adolescents, that is, Assessment Reactivity (AR); second, whether AR predicts treatment outcome. Participants were 177 adolescents with alcohol and other substance use disorders (AOSUD) in a 9-weekly group sessions of cognitive behavioral therapy. Drug urinalysis results at baseline assessment, first and last session and self-report for alcohol use were used as outcome measures. Participants showed a highly significant shift to non-use of both alcohol and of other substances from intake assessment to first session and from first to last session. Alcohol and substance use at first session predicted use at last session. This is the first study in youth asserting Assessment Reactivity as a valid construct during the pretreatment phase. AR should be considered in any analysis targeting treatment outcomes.