Objectives: To determine dementia prevalence and costs attributable to dementia using Veterans Health Administration (VHA) data with and without Medicare data. Data Sources: VHA inpatient, outpatient, purchased care and other data and Medicare enrollment, claims, and assessments in fiscal year (FY) 2013. Study Design: Analyses were conducted with VHA data alone and with combined VHA and Medicare data. Dementia was identified from a VHA sanctioned list of ICD-9 diagnoses. Attributable cost of dementia was estimated using recycled predictions. Data Collection: Veterans age 65 and older who used VHA and were enrolled in Traditional Medicare in FY 2013 (1.9 million). Principal Findings: VHA records indicated the prevalence of dementia in FY 2013 was 4.8 percent while combined VHA and Medicare data indicated the prevalence was 7.4 percent. Attributable cost of dementia to VHA was, on average, $10,950 per veteran per year (pvpy) using VHA alone and $6,662 pvpy using combined VHA and Medicare data. Combined VHA and Medicare attributable cost of dementia was $11,285 pvpy. Utilization attributed to dementia using VHA data alone was lower for long-term institutionalization and higher for supportive care services than indicated in combined VHA and Medicare data. Conclusions: Better planning for clinical and cost-efficient care requires VHA and Medicare to share data for veterans with dementia and likely more generally.