Objective: To establish the prevalence of injecting practices that carry a risk of HIV infection among injection drug users (IDUs) in Indonesia and to examine the risk of sexual transmission of HIV from IDUs to noninjecting populations. Design: A first round of behavioral surveillance among community-recruited male IDUs in 3 cities. Methods: In late 2002, IDU gathering places were mapped in 3 cities, and 650 IDUs were recruited using multiple wave sampling originating from sites systematically selected for diversity. Trained interviewers, mostly former IDUs, administered a questionnaire focusing on injecting practices, sexual behavior, and HIV-related knowledge. Results: Almost all IDUs knew that HIV is transmissible through shared needles, but 85% of injectors reported using a syringe previously used by another IDU in the previous week. Over two thirds of IDUs were sexually active, 48% reported multiple partners, and 40% had bought sex from a female sex worker in the preceding 12 months. Consistent condom use was reported by 10%. Conclusions: The potential for the sexual spread of HIV from IDUs to noninjectors is extremely high in Indonesia. Interventions are urgently needed to reduce high levels of needle sharing, but a focus on needle cleaning and increasing condom use among IDUs is also essential.