Background <p>There is a lack of research on which specific factors that predict positive outcomes in support programs for concerned significant others (CSOs) to individuals with substance use disorders (SUDs). The&#xa0;aim of the&#xa0;study&#xa0;was to investigate predictors of positive outcomes among young adults in the areas of: treatment seeking, decreased substance use, and improved parent-young adult relationships. Outcomes were assessed at 24-weeks following a randomized controlled trial (RCT) comparing Community Reinforcement and Family Training (CRAFT) and manualized counselling for parents of young adults (18-24 years) with hazardous substance use.</p> Methods <p>A secondary analysis from an RCT including 113 parents (92% female) of young adults (87% male) recruited from two outpatient clinics for young adults in Stockholm, Sweden, subsequently nationwide. Clinical and sociodemographic predictors of treatment entry, substance use, and relationship happiness at 24 weeks follow-up were assessed using linear and logistic mixed-effects models.</p> Results <p>Previous young adult treatment engagement and higher parental self-efficacy were significant predictors of treatment entry, explaining 16% of the variance. Baseline alcohol and substance use consistently predicted higher use over time across measures. Parental stress showed complex associations: higher stress predicted reduced substance use in some models but increased use in others. For the measure of relationship happiness, baseline scores were the strongest predictor.</p> Conclusions <p>Taken together, the findings suggest promising targets for tailored support programs for parents of young adults with SUD, such as components designed to enhance parental self-efficacy.</p> Trial registration <p>ISRCTN12212515 <a href="https://www.isrctn.com/ISRCTN12212515">https://www.isrctn.com/ISRCTN12212515</a>. Submitted November 7th, 2018, registered March 4th, 2019.</p>