OBJECTIVES A significant and growing body of literature indicates that positive supportive peer relations are protective of relapse for adolescents treated for psychoactive substance use disorder (PSUD). Unfortunately, no standard measure of peer relations exists. The objective of this research is to use Rasch modeling as an application of item response theory (IRT) to validate a 14-item peer relations scale for use in outcomes assessments in this treatment population. METHODS Subjects are 509 adolescents discharged from primary substance abuse treatment from 2004-2007. The data is from an outcome assessment conducted between 6 and 12 months post discharge via a 234-item questionnaire that included the 14-item peer relations scale. The scale has questions that assess the degree to which the adolescent's peers conform to norms of positive behavior and the degree to which the peers are supportive of abstinence and recovery. Response rate was 62%. RESULTS The person reliability is 0.76 and the Cronbach's alpha person raw score reliability is 0.93 both indicating the scale is a strong metric. The item reliability of 0.99 is high and shows that the model is reliable. The real separation (8.49) meaning items are placed reliably on the Rasch "ruler" with about eight levels of importance identified. The mean-square (MNSQ) statistic of the infit and outfit values were between 0.5 and 1.5 for all of the items indicating a low level of randomness and thus unidimensionality of the scale. Visual inspection of a Wright Item Map shows the hierarchical structure of the scale with a moderate degree of inter-item spread. Similarly, the standardized t-tests (Z-STD) shown on a pathway bubble chart indicate a moderate degree of item overlap. CONCLUSIONS The Rasch IRT analysis shows the peer relations scale is a reliable unidimensional metric of an important treatment outcome for adolescents treated for PSUD.