The Medicare Part D low-income supplement (LIS) is designed to help low-income persons access the prescription drug benefit with reduced premiums and lower out-of-pocket costs. To date, estimates suggest that almost 80% of the persons eligible for the LIS are participating in the program. But despite efforts by CMS and its partners to enroll as many eligible persons as possible, 5.6% of rural Medicare beneficiaries and 3.7% of urban Medicare beneficiaries are estimated to be eligible for but not enrolled in the LIS program. Information in this brief and the accompanying state tables could help CMS partners (including state and county agencies) target counties with high likelihood of identifying and enrolling LIS-eligible beneficiaries. Funded by the Office of Rural Health Policy: Cooperative Agreement for Rapid Response to Issue-Specific Rural Research.