ABSTRACT Screening, Brief Intervention, and Referral to Treatment (SBIRT) research indicates that substance use severity is a predictor of readiness to change, which is in turn associated with positive treatment follow-through. Using a strengths-based approach, family support is hypothesized to be a protective factor related to treatment completion; yet additional research is needed to substantiate the role of this construct when considering other pre-treatment factors. This study used a SEM equation to examine whether self-reported family support mediated the relationship between substance use severity and treatment completion. Service user demographics and pre-treatment characteristics were collected from service users in a primary care setting who were referred to treatment in a statewide implementation of SBIRT. The results provide additional evidence that family support mediates the relationship between symptom severity and substance use treatment completion. The authors conclude with a discussion on how family support can be integrated in SBIRT models for both enhancing the screening process and referrals to treatment.