BackgroundThe epidemiology of intellectual disability co-occurring with schizophrenia and other psychiatric illness is poorly understood. The separation of mental health from intellectual disability services has led to a serious underestimation of the prevalence of dual diagnosis, with clinicians ill-equipped to treat affected individuals.AimsTo estimate the prevalence of dual diagnosis and describe its clinical profile.MethodThe Western Australian population-based psychiatric and intellectual disability registers were cross-linked (total n=245 749).ResultsOverall, 31.7% of people with an intellectual disability had a psychiatric disorder; 1.8% of people with a psychiatric illness had an intellectual disability. Schizophrenia, but not bipolar disorder and unipolar depression, was greatly over-represented among individuals with a dual diagnosis: depending on birth cohort, 3.7-5.2% of those with intellectual disability had co-occurring schizophrenia. Pervasive developmental disorder was identified through the intellectual Disability Register and is therefore limited to individuals with intellectual impairment. None the less, pervasive developmental disorder was more common among people with a dual diagnosis than among individuals with intellectual disability alone. Down syndrome was much less prevalent among individuals with a dual diagnosis despite being the most predominant cause of intellectual disability. individuals with a dual diagnosis had higher mortality rates and were more disabled than those with psychiatric illness alone.ConclusionsThe facility to combine records across administrative jurisdictions has enhanced our understanding of the epidemiology of dual diagnosis, its clinical manifestations and aetiological implications. in particular, our results are suggestive of a common pathogenesis in intellectual disability co-occurring with schizophrenia.Declaration of interestNone. Funding detailed in Acknowledgements.