This study modelled the effects on the prevalence of schizophrenia over the lifespan of Cannabis sativa in eight birth cohorts: 1940–1944, 1945–1949, 1950–1954, 1955–1959, 1960–1964, 1965–1969, 1970–1974, 1975–1979. It derived predictions as to the number of cases of schizophrenia that would be observed in these birth cohorts, given the following four hypotheses: (1) that there is a causal relationship between C. sativa use and schizophrenia; (2) that C. sativa use precipitates schizophrenia in vulnerable persons; (3) that C. sativa use exacerbates schizophrenia; and (4) that persons with schizophrenia are more liable to become regular cannabis users. There has been a steep rise in the prevalence of C. sativa use in Australia over the past 30 years and a corresponding decrease in the age of initiation of use. There was no evidence of a significant increase in the incidence of schizophrenia over the past 30 years. Data on trends for the age of onset of schizophrenia did not show a clear pattern. C. sativa use among persons with schizophrenia has consistently been found to be more common than in the general population. The authors concluded that C. sativa use does not appear to be causally related to the incidence of schizophrenia, but its use may precipitate disorders in persons that are vulnerable to developing psychosis and worsen the course of the disorder among those that have already developed it.