This series of files links two large population-based sources providing detailed data about Medicare beneficiaries with cancer. The SEER (Surveillance, Epidemiology, and End Results) program consists of clinical, demographic, and cause of death information collected from tumor registries beginning in January 1, 1973. The Medicare contribution includes all claims for covered health care services from beneficiaries’ time of eligibility until death. Linkage is processed biennially by SEER and Centers for Medicare and Medicaid Services (CMS) staff. 95% of individuals age 65 and older are included in the SEER files. Due to privacy concerns, access to this database requires an application, SEER-Medicare Data Use Agreement (DUA), and documentation of institutional review board approval. Additionally, the National Cancer Institute’s information technology contractor assesses a processing fee the amount of which is dependent upon the type and number of files requested.