There is an urgent need for a reliable method of evaluating the severity of adolescent chemical abuse and problems related to chemical abuse. The lack of an appropriate rating scale to fill this objective hampers the design and the assessment of objective treatment outcome and follow-up of adolescent chemical abusers. The Teen-Addiction Severity Index (T-ASI) is a structured interview which was developed to assess the seven following domains: chemical use, school status, employment-support status, family relationships, peer-social relationships, legal status, and psychiatric status. This paper discusses the rationale for the design of the T-ASI and presents a preliminary study indicating satisfactory interrater reliability of the rating scale.