BACKGROUND: Early cannabis use has consistently been associated with an increased risk for the later development of psychosis. Studies suggest that Conduct Disorder (CD) is more common amongst young people who later go on to develop psychosis. CD has been associated with greater and earlier cannabis use in general population samples. Based on this evidence, we hypothesised that among patients experiencing their first episode of psychosis, the presence of CD symptoms prior to age 15 would be associated with cannabis use. METHOD: 102 patients experiencing a first episode of psychosis were interviewed to assess CD symptoms prior to age 15 and use of cannabis and other substances. RESULTS: The number of CD symptoms was significantly associated with lifetime cannabis use (odds ratio=5.41 (1.76-16.57), p=0.03) and with first use of cannabis before age 14 (odds ratio=1.46 (1.12-1.92), p=0.006), after controlling for stimulant/hallucinogen use and level of education. CONCLUSIONS: Among patients experiencing a first episode of psychosis, CD symptoms were significantly associated with use of cannabis and with use by age 14. Among individuals vulnerable for psychosis, CD symptoms may independently increase the likelihood of cannabis use which in turn increases the risk of psychosis.