This study examined emergency department use among Medicare beneficiaries with serious mental disorders. Drawn from the 2004 national 5% sample of Medicare beneficiaries, the sample in this study (N=129,805) included fee-for-service enrollees with primary diagnoses of schizophrenia, major depression, other major affective disorders, or other psychoses. Emergency department use was compared by psychiatric diagnostic category and presence of a claim related to substance use disorder. Having any emergency department visit was common for beneficiaries aged >or=65 years (58.3%) and beneficiaries <65 years who were Medicare eligible because of a disability (48.5%). Emergency department visits with a primary psychiatric diagnosis occurred for 14.8% of disabled beneficiaries and 6.7% of aged beneficiaries. Use varied by diagnosis and was higher for those with any substance-related claims (p<.01). Emergency department use was common in the sample. Within each eligibility category, use varied by psychiatric diagnostic category and presence of a substance-related claim.