Increasingly, national and state health and other registers are being made available for research use. Record linkage across these registers is a cost-effective and efficient means of acquiring large samples and overcoming some of methodological issues facing researchers studying rare outcomes. This presentation describes the methods and benefits of record linkage in the Australian context. To illustrate the points made, it focuses on two studies using cross-linked data from Western Australian population-based registers that are examining the association between schizophrenia and intellectual disability. Although these are rare disorders, both with a lifetime population prevalence of ˜1%, the prevalence of schizophrenia among persons with an intellectual disability is reportedly three times the population estimate. However, little is known about the relationship between the two disorders. In the first study, cross-linked data from the intellectual disability register and the psychiatric case register are used to get estimates of the prevalence of comorbidity and to describe the profile of persons with a dual diagnosis of intellectual disability and psychiatric illness, particularly the psychoses. In the second, data from linkage across multiple registers including psychiatric, midwives and intellectual disability registers are used to compare the risk of intellectual disability in children of mothers with schizophrenia and affective psychoses with children of mothers with no recorded psychiatric history. The data are suggestive of underlying genetic risk factors of varying diagnostic specificity. These findings are presented and their etiological implications discussed. The presentation concludes that the facility to integrate records across multiple population databases, sometimes held in separate administrative jurisdictions, has had a marked impact on our capacity to estimate the extent of comorbidity and its prevalence across diagnostic categories, to understand its clinical manifestations and to start to untangle etiological underpinnings.