The best time to enroll in Medigap is during your Medigap open enrollment period. This is a six-month period that begins the month that you are enrolled in Medicare Part B and are at least 65 years old (or qualify because of a disability or end-stage renal disease). During your open enrollment period, insurance companies are not allowed to use medical underwriting when considering your application. You may also switch Medigap plans during this time if you aren't satisfied with the one you initially selected. However, if you enroll in a plan outside of your open enrollment period, insurance companies may be allowed to base the cost of your plan on your current and past health. This could lead to a higher premium. And if a company determines your health is too poor, they may deny you coverage entirely (which they are not allowed to do if you enroll during your open enrollment period).