This paper reports on the prevalence of drug use in Asia and the Pacific. It is based on the report "Situational analysis of illicit drug issues and responses in Asia and the Pacific", commissioned by the Australian National Council on Drugs Asia Pacific Drug Issues Committee. Review of existing estimates of the prevalence of people who use illicit drugs from published and unpublished literature and information from key informants and regional institutions was undertaken for the period 1998 - 2004. Estimates of the prevalence of people who use illicit drugs were conducted for 12 Asian and six Pacific Island countries. The estimated prevalence of those using illicit drugs ranges from less than 0.01% to 4.6%. Countries with estimated prevalence rates higher than 2% are Cambodia, Hong Kong, Philippines, Thailand, Indonesia, Laos and Malaysia. China, Myanmar and Vietnam have estimated prevalence rates ranging between less than 0.01% and 2%. Data to estimate prevalence rates was not available for Pacific Island countries and Brunei. Estimates of the prevalence of drug use are critical to policy development, planning responses and measuring the coverage of programs. However, reliable estimates of the numbers of people using illicit drugs are rare in Asia, particularly the Pacific.