Adults with intellectual disability or autism are at risk of psychiatric admission which carries personal, social and economic costs. We identified 654 adults with intellectual disability or autism in the electronic clinical records of one mental health trust. We investigated the demographic and clinical factors associated with admission and readmission after discharge. Young male patients with intellectual disability, schizophrenia and previous admissions are most at risk of the former, whereas affective and personality disorders predict the latter. Both community intellectual disability services and mental health crisis care must focus on providing effective support for those patients.