Childhood immunization rates can be used as a barometer of the quality of care delivered to children. Medicaid managed care has the potential to improve childhood immunization delivery by implementing population-based medicine in primary care. Studies of immunization rates under Medicaid managed care have found mixed results; in some settings immunization rates were higher for children covered by Medicaid managed care than for children covered by fee-for-service Medicaid, but in other settings immunization rates were similar for children in the two types of Medicaid programs. In most studies, overall rates were well below national guidelines for all populations enrolled in Medicaid.