ABSTRACT. Objective: The aim of this research was to study marijuana use, associated risks, and response to brief motivational intervention among young adult drinkers treated in an emergency department. Method: Study participants (N = 215; ages 18-24) were in a randomized controlled trial for alcohol use that compared motivational interviewing with personalized feedback (MI) with personalized feedback only. Pastmonth marijuana users were compared with nonusers on demographics, readiness, self-effi cacy, and behavioral risk variables. Marijuana use was examined as a potential moderator of alcohol outcomes. Whether marijuana use alone or combined marijuana and alcohol use would be reduced as a result of brief intervention for alcohol was examined at 6 and 12 months. Results: Current marijuana users were younger, were more likely to be white, and reported more alcohol use, other illicit drug use, and more alcohol-related consequences than nonmarijuana users. Marijuana use at baseline did not moderate response to brief alcohol treatment. Marijuana use declined from baseline to 6 months for both treatment groups, but only MI participants continued to reduce their use of marijuana from 6-to 12-month follow-up. Reductions in number of days of use of marijuana with alcohol appeared to be primarily a function of decreased alcohol use. Conclusions: Young adult drinkers reporting current marijuana use are at generally higher risk but responded to brief alcohol treatment by reducing alcohol and marijuana use. , 1988) and in adult ED patients treated for alcohol problems (Woolard et al., 2003); and greater injury-related risk has been shown among patients using both alcohol and marijuana (Soderstrom et al., 1988;Woolard et al., 2003).Medical settings provide an opportunity for screening (Chung et al., 2003) and early intervention with non-treatment-seeking alcohol and marijuana users (Degutis, 2003). ED studies of brief motivational interventions (BMIs) for alcohol have demonstrated positive outcomes with older adolescent and young adult samples (Monti et al., 1999(Monti et al., , 2007. Although studies have effectively targeted multiple risk behaviors for injury among adolescents in the ED (Johnston et al., 2002), we found no published studies that targeted multiple substances with older adolescents or young adults in ED settings. However, BMI has reduced both alcohol and marijuana use among adult ED patients more than standard services (Woolard et al., 2008). A few studies have targeted multiple substances with college students, and a recent study examined adolescent substance use in a general medical setting. D'Amico and colleagues (2008) found greater reduction of marijuana use in a BMI compared with usual services among at-risk adolescents recruited in a primary care clinic. BMI and written personalized feedback were effective in reducing alcohol, nicotine, and marijuana use with mandated college students (White et al., 2006(White et al., , 2007, and non-help-seeking college students decreased alcohol, nicotine, and ...