Sealed radioactive sources, radioactive material encapsulated in stainless steel or other metal, are used worldwide in medicine, industry, and research. These sealed sources pose a threat to national security because terrorists could use them to make “dirty bombs.” GAO was asked to determine (1) the number of sealed sources worldwide and how many have been reported lost, stolen, or abandoned; (2) the controls, both legislative and regulatory, used by countries that possess sealed sources; and (3) the assistance provided by the Department of Energy (DOE) and other U.S. federal agencies to strengthen other countries’ control over sealed sources and the extent to which these efforts are believed to be effectively implemented.The precise number of sealed sources in use is unknown because many countries do not systematically account for them. However, nearly 10 million sealed sources exist in the United States and the 49 countries responding to a GAO survey. There is also limited information about the number of sealed sources that have been lost, stolen, or abandoned, but it is estimated to be in the thousands worldwide. Many of the most vulnerable sealed sources that could pose a security risk are located in the countries of the former Soviet Union. All of the 49 countries that responded to GAO’s survey reported that they have established legislative or regulatory controls over sealed sources. However, nuclear safety and security experts from DOE, the Departments of State and Defense, the Nuclear Regulatory Commission (NRC), the International Atomic Energy Agency, and the European Commission told GAO that countries’ controls over sealed sources vary greatly and are weakest among less developed countries. In fiscal year 2002, DOE established a program focusing on improving the security of sealed sources in the former Soviet Union and has started to fund security upgrades in Russia and other former Soviet countries. The Departments of Defense and State and NRC also have programs to help countries strengthen controls over sealed sources. DOE plans to expand its program to other countries and regions in 2003 and is developing a plan to guide its efforts. However, the department has not fully coordinated its efforts with NRC and the Department of State to ensure that a government-wide strategy is established. In addition, as of January 2003, the majority of DOE’s program expenditures totaling $8.9 million were spent by DOE’s national laboratories in the United States.