This article analyzes a change in "Medigap" regulations that occurred in Missouri in 1999. It allows Medicare beneficiaries in the state to switch to a different carrier each year so long as they retain the same standardized policy type, without losing their open enrollment privileges. The analysis is based on a comparison of various outcomes in Missouri and those in two comparison states, Kansas and Florida. We found little evidence that the policy change affected premiums charged by insurance carriers in Missouri, but conclude that other desirable aspects of the change make it potentially attractive for other states to follow.