Substance Use, Resulting Disorders, and Collateral Mental Disorders Among Adolescents in a Special Outpatient Institutions for Addictions Abstract. Objective: Only few clinics offer the outpatient treatment of substance use disorders (SUDs) among adolescents. Therefore, only limited data describe substance use patterns, SUDs, and co-occurring psychiatric disorders characteristic of adolescents who present in such outpatient clinics specialized in the treatment of SUDs. Method: Via interview we collected data from n = 201 patients between 12 and 19 years concerning their substance use, SUDs, and current co-occurring psychiatric disorders. We created descriptive presentation of data regarding use patterns, SUDs, and co-occurring disorders divided by sex and current age. Results: Tobacco (88 %) and cannabis (86 %) were the most frequently used substances. 67 % of all patients presented with more than one SUD, cannabis use disorder being the most prevalent one (84 %). 72 % presented with at least one co-occurring disorder, with conduct disorders (40 %), attention deficit (hyperactivity) disorders (21 %), and depressive disorders (18 %) being the most frequent ones. Conclusions: Adolescent SUD patients often present with co-occurring psychiatric disorders. Institutions for adolescent SUD treatment should also focus on treating co-occurring conduct disorders, depression, and attention deficit disorders.