The National Cancer Institute and the Health Care Financing Administration share a strong research interest in cancer costs, access to cancer prevention and treatment services, and cancer patient outcomes. To develop a database for such research, the two agencies have undertaken a collaborative effort to link Medicare Program data with the Surveillance, Epidemiology, and End Results (SEER) Program database. The SEER Program is a system of 9 population-based tumor registries that collect standardized clinical information on cases diagnosed in separate, geographically defined areas covering approximately 10% of the US population. Using a deterministic matching algorithm, the records of 94% of SEER registry cases diagnosed at age 65 or older between 1973 to 1989, or more than 610,000 persons, were successfully linked with Medicare claims files. The resulting database, combining clinical characteristics with information on utilization and costs, will permit the investigation of the contribution of various patient and health care setting factors to treatment patterns, costs, and medical outcomes.