In Jakarta, Malang, Semarang and Medan, IDUs were selected through two-stage, time-location sampling. Lists of venues where IDUs congregated (streets, parks, private houses and drop-in) were developed by non-governmental organizations providing services to IDUs and local health authorities. Samples of venues were chosen via systematic-random sampling with probability proportional to venue size. All IDUs present at the time of data collection were selected for participation.