Our first objective was to develop an index of satellite exchange and then determine whether satellite exchangers (SEs) differed demographically or behaviorally from other injecting drug users (IDUs). Our second objective was to determine the degree that SEs contributed to needle exchange program (NEP) effectiveness.We collected data from approximately 5000 Baltimore Needle Exchange Program (BNEP) participants on the number of syringes acquired and returned over the two-year period February 1995 to February 1997. We then conducted one-way ANOVAs and logistic regressions to determine if SEs were different from other IDUs.We classified 9.35% of the IDUs and SEs and showed that SEs reported levels of drug use and risk behavior similar to other BNEP participants. Although SEs represented less than 10% of all BNEP clients, they accounted for more than 64% of all needles distributed by the BNEP. We showed that SEs accessed more wide-ranging drug use networks than non-SE IDUs and thus can act as potential bridges for human immunodeficiency virus (HIV) prevention materials and messages to larger numbers of drug injectors.SEs can be expressly targeted with specific prevention messages and encouraged to be "ambassadors" for HIV prevention messages. Efforts to curtail the activities of SEs may detract from the effectiveness of NEPs.