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Aims: To test the hypothesis that among non-treatment-seeking emerging adults (EA) who both use marijuana and have alcohol binges, a brief, longitudinally delivered, developmentally based motivational intervention would show greater reductions in the use of these two substances compared with a health education control condition., Design: Parallel, two-group, randomized controlled trial with follow-up interventions conducted at 1, 3, 6 and 9 months and final assessments at 12 and 15 months., Setting: Hospital-based research unit in the United States., Participants: Community-based 18-25-year-olds who reported at least monthly binge drinking and at least weekly marijuana use., Intervention: Motivational intervention (EA-MI) focused primarily on themes of emerging adulthood (identity exploration, instability, self-focus, feeling in-between, a sense of possibilities) and the subjects' relationship to substance use (n = 110) compared with an attention-matched health education control condition (n = 116)., Measurements: The primary outcomes were days of binge alcohol, marijuana and dual use day as measured using the timeline follow-back method analysing the treatment by time interaction to determine relative differences in the rate of change between intervention arms., Findings: At baseline, the mean rate (days/30) of binge drinking was 5.23 (± 4.31) of marijuana use was 19.4 (± 10.0) and of dual (same day) use was 4.11 (± 4.13). Relative to baseline, there were reductions in the rate of binge alcohol use, marijuana use and days of combined binge alcohol and marijuana use (P < 0.001) at all follow-up assessments. However, the treatment × time interaction was not statistically significant for alcohol (P = 0.37), for marijuana (P = 0.07) or for dual use (P = 0.55). Averaged over all follow-ups, mean reductions in binge, marijuana and dual use days were 1.16, 1.45 and 1.08, respectively, in the health education arm, and 1.06, 1.69 and 0.96 in EA-MI. Bayes factors were < 0.01 for frequency of binge alcohol use and frequency of dual binge alcohol and marijuana and 0.016 for marijuana use., Conclusions: A brief, longitudinally delivered, developmentally based motivational intervention for young adults did not produce reductions in binge alcohol, marijuana use or dual use days relative to a control condition., (© 2017 Society for the Study of Addiction.)