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To test whether beneficial effects of a single session of Motivational Interviewing (MI) on alcohol, tobacco and illicit drug use apparent after 3 months were maintained until 12 months.Cluster randomized trial, allocating 200 young people in the natural groups in which they were recruited to either MI (n = 105) or to an assessment-only control condition (n = 95).Ten further education colleges across inner London.Two hundred young people who were current users of illegal drugs (age range 16–20 years) with whom contact was established through peers trained for the project.The intervention was adapted from MI in the form of a topic-based 1-hour single-session discussion.Changes in cigarette, alcohol, cannabis and other drug use and perceptions of risk and harm between the time of recruitment and follow-up interviews after 3 and 12 months.A satisfactory follow-up rate (81%) was achieved. After 12 months, 3-month differences between MI and assessment-only groups have disappeared almost entirely. Unexpected improvements by the assessment-only control group on a number of outcomes suggest the possibility of reactivity to the research assessment at 3-month follow-up.In the terms of the original experiment, there is little evidence of enduring intervention effectiveness shown by between-group differences after 12 months. Deterioration of effect is the most probable explanation, although reactivity to 3-month assessment, a late Hawthorne effect, cannot be ruled out.