This study was designed in a cluster-randomized trial, allocating 200 young people in the natural groups in which they were recruited to either motivational interviewing or a non-intervention education-as-usual control condition. The setting of the study included 10 further education colleges across inner London. The participants were 200 young people currently using illegal drugs, with which contact was established through peers trained for the project. The intervention was adapted from the literature on motivational interviewing in the form of a single 1 hour session face-to-face interview structured by a series of topics. Changes in self-reported cigarette, alcohol, cannabis, and other drug use were measured in a range of drug specific perceptions and other indicators of risk and harm. Measurement took place at recruitment and the follow-up interview 3 months later. A strong follow-up rate of 89.5 percent was achieved. In comparison to the control group, those randomized to motivational interviewing reduced their cigarette, alcohol, and cannabis use mainly through moderation rather than cessation. Effect sizes were 0.37, 0.34, and 0.75 for reductions in the use of cigarettes, alcohol and cannabis, respectively. Change was also evident in the various indicators of risk and harm, but not as widely in the changes in drug compensation. This study provides the first substantial evidence of non-treatment benefit to be derived among young people involved in illegal drug use in receipt of motivational interviewing. The targeting of multiple drug use in generic fashion among young people has also been supported. Therefore, the study was successful in reduction of drug use among young people when a single session motivational interviewing approach was utilized. Tables, references