This case study from the UNAIDS/WHO Working Group on Global HIV/AIDS/STI Surveillance recalls the reasons for making national estimates of HIV infection, and goes on to present the difficulties posed by concentrated epidemics and the challenges faced by Indonesia. It discusses how to decide on a process and put it into practice, and shares lessons learnt from the work in Indonesia. The study concludes that Indonesia’s experience demonstrates that it is possible to make such estimates in a huge, geographically, culturally and epidemiologically diverse country. Other countries with HIV epidemics concentrated in populations with high risk behaviour can benefit from Indonesia’s experience by adopting a similar estimation process.