Beginning January 2006, Medicare ben-eficiaries will have limited ability to changehealth plans. We examine the Medicaremanaged
care enrollment and disenroll-ment behavior of traditionally vulnerablebeneficiaries from 1999-2001 to estimatethe potential
impact of the new enrollmentrestrictions. Findings that several suchgroups were more likely to make multiplehealth plan elections,
leave their managedcare plan midyear, and/or have higher vol-untary disenrollment rates and transfers tooriginal fee-for-service
(FFS) Medicaresuggest that the lock-in provisions may havegreater negative impacts on vulnerable ben-eficiaries. This article
identifies several rec-ommendations that CMS might consider tolessen the detrimental effects on at-riskgroups.