There is concern that churning in Medicaid excludes children from the accountability system for managed care because they may not meet the one-year continuous enrollment requirement. This study explores the effect of churning in measuring childhood immunization coverage rates under the current accountability system. Data were collected from administrative databases at the Centers for Medicaid and Medicare Services and 12 states with high Medicaid managed care penetration. On average in the 12 states only 39% of the children enrolled in one specific managed care plan met the continuous enrollment requirement. However, Centers for Medicaid and Medicare Services data showed that 78% of children were enrolled in Medicaid (but not the same plan) continuously for 12 months. Both plan-specific rates and overall Medicaid rates varied greatly across the states. Policies that result in churning mean that many vulnerable children fall outside of the accountability structure intended to assure that they receive necessary services.