This longitudinal study examined whether youth with high traumatic stress (HTS) respond differently to outpatient substance abuse treatment compared to youth without HTS at intake, and at 3 and 6 months following intake. Data were analyzed using a mixed-effects two-part model to fit repeated data with zero saturation. Clients in both groups significantly increased their odds of abstinence and full symptom remission of substance problems from baseline to 6 months. Of youth still using substances, the group with HTS reduced substance use significantly more than substance users without HTS. Recommendations for future research are provided.