Cannabis use is approximately twice as high among people with schizophrenia as among the general population. Evidence for cannabis use predisposing to psychoses later in life came many years ago from a study of Swedish conscripts. A dose-response relationship was observed between cannabis use at conscription and diagnosis of schizophrenia 15 years later. In 2002, similar findings were reported from The Netherlands where cannabis use was found to increase the risk of psychosis in psychosis-free individuals. A birth cohort study from Christchurch examined the relationship between cannabis use and the development of schizophrenia. Individuals who were cannabis dependent at age 18 years had a 3.7-fold increased risk of psychotic symptoms than those who were not cannabis dependent. Furthermore, the development of psychotic symptoms tended to decrease the consumption of cannabis. The Dunedin study showed that individuals using cannabis at ages 15 and 18 years had increased rates of developing psychotic symptoms, and carriers of the COMT val allele were most likely to develop schizophreniform psychosis after adolescent cannabis use street drug users know that cannabis can induce delusions (though not hallucinations). There is also some preliminary evidence that one of the reasons for the increase in the incidence of schizophrenia is south London is the increased consumption of cannabis. Our most recent studies concern the mechanism of action of cannabis.