Abstract: Syringe-exchange programs (SEPs) will likely play a major role in slowing the spread of acquired immunodeficiency syndrome (AIDS) among injecting drug users (IDUs), but the success of any single SEP will depend to a large extent on where it is located. We show how the optimal position for a new SEP can be chosen given accurate knowledge of where IDUs live and how far they are willing to travel to an SEP. This information is not normally available, and one of our major points is that SEPs will necessarily be placed in suboptimal locations and will serve fewer IDUs than they otherwise might until it becomes available. Our method for choosing the best SEP placement is illustrated with Manhattan as an idealized example.