BACKGROUND National epidemiological surveys have reported increased rates of cannabis use and cannabis use disorders (CUDs) among individuals with mental illness. However, this subject has not been sufficiently investigated, particularly given limitations in diagnostic tools used and lack of data pertaining to frequency of cannabis used. OBJECTIVE To examine the prevalence of cannabis use and CUDs among individuals with a wide range of mental illness. METHODS We analyzed data on 43,070 respondents age 18 and above from the National Epidemiologic Survey on Alcohol and Related Conditions, a nationally representative survey conducted from 2001 to 2002. Main outcome measures included rates of cannabis use by frequency (at least weekly and less than weekly use) and DSM-IV CUDs according to the number and type of axis I and axis II psychiatric diagnoses, assessed by the Alcohol Use Disorders and Associated Disabilities Interview Schedule-IV. We estimated the proportion of cannabis used by individuals with mental illness using reported daily dose and frequency of cannabis used by individuals with and without mental illness. RESULTS Rates of weekly cannabis use, less than weekly cannabis use and CUDs among individuals with 12-month mental illness were 4.4%, 5.4% and 4.0%, respectively, compared to 0.6%, 1.1% and 0.4%, respectively, among individuals without any 12-month mental illness (P<0.0001 for all comparisons). The odds ratio for cannabis use among individuals with 12-month mental illness vs. respondents without any mental illness was 2.5, and the odds of having a CUD among individuals with 12-month mental illness were 3.2, after adjusting for sociodemographic variables and additional substance use disorders. Cannabis use and CUDs were particularly associated with bipolar disorder, substance use disorders and specific (anti-social, dependant and histrionic) personality disorders. Persons with a mental illness in the past 12 months represented 72% of all cannabis users and we estimated they consumed 83% of all cannabis consumed by this nationally representative sample. CONCLUSIONS The current study provides further evidence of the strong association between cannabis use and a broad range of primary mental illness. This emphasizes the importance of proper screening for frequent cannabis use and CUDs among individuals with primary mental illness and focusing prevention and treatment efforts on this population.