BackgroundIntellectual and developmental disabilities (IDDs) and psychiatric disorders frequently co-occur. Although each has been associated with negative outcomes, their combined effect has rarely been studied.AimsTo examine the likelihood of five negative health and healthcare outcomes for adults with IDD and mental health/addiction disorders (MHAs), both separately and together. For each outcome, demographic, clinical and system-level factors were also examined.MethodLinked administrative data-sets were used to identify adults in Ontario, Canada, with IDD and MHA (n = 29 476), IDD-only (n = 35 223) and MHA-only (n = 727 591). Five outcomes (30-day readmission, 30-day repeat ED visit, delayed discharge, long-term care admission and premature mortality) were examined by logistic regression models with generalised estimating equation or survival analyses. For each outcome, crude (disorder groups only) and complete (adding biosocial covariates) models were run using a general population reference group.ResultsThe IDD and MHA group had the highest proportions across outcomes for both crude and complete models. They had the highest adjusted ratios for readmissions (aOR 1.93, 95%CI 1.88–1.99), repeat ED visit (aOR 2.00, 95%CI 1.98–2.02) and long-term care admission (aHR 12.19, 95%CI 10.84–13.71). For delayed discharge, the IDD and MHA and IDD-only groups had similar results (aOR 2.00 (95%CI 1.90–2.11) and 2.21 (95%CI 2.07–2.36). For premature mortality, the adjusted ratios were similar for all groups.ConclusionsPoorer outcomes for adults with IDD, particularly those with MHA, suggest a need for a comprehensive, system-wide approach spanning health, disability and social support.