Purpose: Little is known about mental health disorders (MHDs) and their associated health care expenditures for the dual eligible elders across long-term care (LTC) settings. We estimated the 12-month diagnosed prevalence of MHDs among dual eligible older adults in LTC and non-LTC settings and calculated the average incremental effect of MHDs on medical care, LTC, and prescription drug expenditures across LTC settings. Methods: Participants were fee-for-service dual eligible elderly beneficiaries from 7 states. We obtained their 2005 Medicare and Medicaid claims data and LTC program participation data from federal and state governments. We grouped beneficiaries into non-LTC, community LTC, and institutional LTC groups and identified enrollees with any of 5 MHDs (anxiety, bipolar, major depression, mild depression, and schizophrenia) using the International Classification of Diseases Ninth Revision codes associated with Medicare and Medicaid claims. We obtained medical care, LTC, and prescription drug expenditures from related claims. Results: Thirteen percent of all dual eligible elderly beneficiaries had at least 1 MHD diagnosis in 2005. Beneficiaries in non-LTC group had the lowest 12-month prevalence rates but highest percentage increase in health care expenditures associated with MHDs. Institutional LTC residents had the highest prevalence rates but lowest percentage increase in expenditures. LTC expenditures were less affected by MHDs than medical and prescription drug expenditures. Implications: MHDs are prevalent among dual eligible older persons and are costly to the health care system. Policy makers need to focus on better MHD diagnosis among community-living elders and better understanding in treatment of MHDs in LTC settings.