OBJECTIVES: Undertreatment has been a prevalent problem among patients with serious mental illnesses (SMI), including bipolar disorder, schizophrenia, and depressive disorders. However, previous research has focused on patients under the age of 50. This study aims to quantify pharmacological treatment incidence among older adults with SMI and visualize associated regional variation. METHODS: Using 2006-2012 Medicare administrative and prescription drug claims data, this retrospective cohort study identified fee-for-service beneficiaries aged over 64 and newly-diagnosed with SMI (based on associated ICD-9-CM diagnostic codes recorded in one inpatient or two non-drug claims and a two-year reference period). Pharmacological treatment initiation was defined as any prescription fill for medications indicated for the earliest SMI condition in the 12 months after diagnosis (no evidence of prior use in the 6 months before initiation). Prevalent users were excluded. Generalized linear mixed models were conducted to estimate regional pharmacological treatment incidences at the hospital referral region level, adjusting for beneficiary demographics, comorbidity, health services utilization, and regional physician supply. Both crude and adjusted regional pharmacological treatment incidences were examined with global spatial randomness and spatial clustering using Moran’s I and local indicator of spatial autocorrelation (LISA) respectively. RESULTS: The sample included 37,448 beneficiaries with at least one newly-diagnosed SMI condition in 2008-2011. Almost 65% initiated pharmacological treatment during the follow-up period. Moran’s I results suggested both crude (I=0.19) and adjusted (I=0.36) regional pharmacological treatment incidences were spatially non-random (p<.0001). LISA results returned highly localized regional pharmacological treatment incidences, with hot spots clustering in Midwest and upper Pacific West and cold spots in West South Central and lower New England after adjustment. CONCLUSIONS: Findings suggest the majority of older adults with SMI receive pharmacological treatment after SMI diagnosis, although treatment initiation rates vary by location. Further investigation examining contributing factors and consequences of geographic variation is required.