This document summarizes preliminary results from linking 2008 Medicaid enrollment data (MSIS) to health insurance response data from the 2008 American Community Survey (ACS). The key findings are: 1) Around 75.9% of individuals enrolled in Medicaid according to MSIS were reported as being on Medicaid in the ACS, implying an undercount of Medicaid enrollment of around 24.1%. 2) The undercount was higher for older age groups and individuals with higher family incomes. 3) This level of undercount in the ACS could result in an overestimation of the uninsured rate in the US by around 1.2 percentage points nationally.