An extensive literature documents geographic variations in patterns of health services use and spending but virtually ignores mental health services. The authors assessed geographic variations in use of and spending on mental health services among adult disabled Medicaid recipients with mental illness. Data were derived from 2008 Medicaid claims in 35 states. Per capita annual inpatient days, ambulatory visits, psychotropic medication fills, and spending on psychiatric services varied widely across regions. The proportion of total variation explained by interstate differences ranged from 43% (inpatient days) to 71% (ambulatory visits). Understanding these variations more thoroughly may help improve the effectiveness and efficiency of mental health services delivered under Medicaid.