Medicare will begin denying claims on May 1, 2013, if the ordering/referring provider listed on the claim is not in the Provider Enrollment, Chain and Ownership System (PECOS), the database Medicare uses to track physicians and other providers. If you bill Medicare, you are encouraged to note any Medicare EOBs with the remittance code N264 and/or N265, which may indicate that the ordering/referring provider on the claim is not yet in PECOS. These providers must take action to enroll in PECOS or future claims that you submit with these providers listed for dates of service on or after May 1 will be rejected. Physicians and other providers are not required to enroll with Medicare to provide services in order to be listed in PECOS. Physicians who wish to be listed in PECOS solely for ordering/referring purposes may submit an enrollment application online via the PECOS website or by completing a CMS-855o paper enrollment application. Click here to access the PECOS system to determine if you or another physician is currently enrolled in PECOS . Click here to download a paper CMS-855o application . Click here for the MLN Matters article for details about the update on PECOS and ordering/referring.