No international agreement currently requires reporting the loss of control of even the most dangerous radioactive sources and materials. Rising concerns about the potential use of these materials by terrorists or criminals (for example, by using explosives to disperse radioactivity with a “dirty bomb”), as well as the potential for international movement of materials, makes the loss of control of strong radioactive sources and materials a vital concern to all countries. This article makes the case for a binding international agreement that would mandate international reporting of the loss of control of the two highest-strength categories of sources and materials, as defined by the International Atomic Energy Agency. No current agreements meet the need for mandatory reporting, although the underlying basis for an agreement already exists. The burdens of such an agreement on any state would be minimal, and all states should be interested in pursuing such an agreement. Agreement to a binding international convention would be an ideal state “gift” for the final Nuclear Security Summit, scheduled to take place in Washington in 2016.