Background and objective: Hard to reach populations such as injecting drug users (IDU) is one, among the most at risk, for
HIV infection in Thailand. This study examined an indirect method for estimating the size of IDU population in Thailand.
Material and Method: A household survey was conducted among 3,790 individuals who were 12-65 years old, stayed at least 3 months in the sample households. Through, reference groups and summation procedure, personal network size was
estimated. The participants were asked to identify the number of their acquaintances whom they perceived to be IDU. Using the survey results with the network scale-up method, the IDU population size was estimated.
Results: The personal network size was estimated to be 71,000 IDUs. The estimated prevalence of total population in Thailand was 110 IDUs in 100,000 population.
Conclusion: Estimating population sizes through the network scale-up method appeared to be an effective method in terms
of time, simplicity, and low cost as compared with more-conventional methods.

Keywords: Injecting drug users (IDU), Network scale-up method (NSU), Size estimation, Thailand