Abstract: Background: Indonesia faces an HIV epidemic that is in rapid transition. Injecting drug users (IDUs) are among the most heavily affected risk populations, with estimated prevalence of HIV reaching 50% or more in most parts of the country. Although Indonesia started opening methadone clinics in 2003, coverage remains low.Methods: We used the Asian Epidemic Model and Resource Needs Model to evaluate the long-term population-level preventive impact of expanding Methadone Maintenance Therapy (MMT) in West Java (43 million people). We compared intervention costs and the number of incident HIV cases in the intervention scenario with current practice to establish the cost per infection averted by expanding MMT. An extensive sensitivity analysis was performed on costs and epidemiological input, as well as on the cost-effectiveness calculation itself.Results: Our analysis shows that expanding MMT from 5% coverage now to 40% coverage in 2019 would avert approximately 2400 HIV infections, at a cost of approximately US$7000 per HIV infection averted. Sensitivity analyses demonstrate that the use of alternative assumptions does not change the study conclusions.Conclusion: Our analyses suggest that expanding MMT is cost-effective, and support government policies to make MMT widely available as an integrated component of HIV/AIDS control in West Java.