Aims: To evaluate the efficacy of a telephone-based intervention consisting of four sessions of motivational interviewing (MI) and cognitive behavioural therapy (CBT) designed to assist individuals to reduce their cannabis use and related problems. Design: Random allocation to intervention or delayed treatment control with 4- and 12-week follow-up assessments. Setting: Counsellors from the Cannabis Information and Helpline (CIH), an Australian reactive telephone service, delivered the intervention to callers seeking treatment. Participants: A total of 160 participants were recruited by the CIH, with 110 participants completing the final follow-up assessment (69% retention). Measurements: Cannabis use, dependence and related problems and other substance use were assessed at baseline and follow-up. Findings: Intervention participants reported greater reductions in dependence symptoms [P<0.001, d=0.9 (0.5-1.3)] and related problems [P<0.001, d=0.5 (0.1-0.9)] compared with control participants at both follow-up assessments. Compared with control, intervention participants reported greater confidence to reduce cannabis use at 4 weeks [P=0.002, d=0.5 (0.1-0.9)], and in turn reported a greater percentage of abstinent days at 12 weeks [P=0.019, d=0.6 (0.2-1.0)]. Conclusions: A brief course of motivational interviewing plus cognitive behavioural therapy delivered by telephone can help to reduce cannabis dependence and promote abstinence in the short term.