This study investigated associations between the presence of developmental disabilities and length of inpatient stay for mental health care. All psychiatric admissions of people with developmental disabilities over a 5-year period were selected (n = 294), and were compared using survival analysis to a random sample of admissions from the general psychiatric population (n = 287). Overall, people with developmental disabilities stayed in hospital longer than those without developmental disabilities, and this extra stay was partially attributed to casemix differences between the cohorts. Subanalysis in both cohorts showed that those going back to their usual living arrangement stayed a shorter period than those who were discharged elsewhere, and that people with developmental disabilities were less likely to be discharged to their usual living arrangement than were people without the disability. This study highlighted the importance of specialized residential and personal supports for people with developmental disabilities and a coexisting mental disorder.