Cross-sectional studies show that a small percentage of children consume a large portion of the health care dollar. However, little is known about the persistence of high health care expenditures among children enrolled in Medicaid health maintenance organizations (HMOs) or in primary care case management (PCCM) programs. This study found substantial expenditure persistence during a 3-year period. Among Temporary Aid to Needy Families (TANF) children in HMOs in the top 10 percent of the expenditure distribution in 2003, 34 percent remained in the top 10 percent in 2004, and 23 percent remained in the top 10 percent in 2005. Similar patterns were observed for TANF children in the PCCM. Expenditure persistence varied based on the enrollees' enabling and need characteristics, providing opportunities to identify children with high health care expenditures who may benefit from care coordination.