Thailand has the most mature HIV/AIDS epidemic in the East Asia and Pacific region. By the mid-1990s, overall prevalence in some parts of the country, particularly in the far north, had reached six or seven percent, and HIV/AIDS had established itself firmly among the general population. In 2000, an estimated 2.3% of the adult population was living with HIV/AIDS.

From early in the 1990s, Thailand mounted a broad, collaborative and committed multisectoral response. In 1996, overall new HIV infections dropped for the first time, and have been static or gradually falling since then – an achievement matched by only two other developing countries in the 20-year history of the global pandemic of HIV/AIDS. While the cumulative number of Thai people who have been infected remains high – at more than one million – it is much smaller than the figure projected in the early 1990s. Some 63% of the national HIV/AIDS budget is committed to treatment of people already infected.

The worst impacts of Thailand's epidemic have been in the mountainous Upper North of the country. It is an economically diverse region, home to some of the richest and very poorest people in Thailand, including numerous tribal groups and often undocumented migrants from Myanmar and China. The Upper North and the Northeast are the biggest domestic source of sex workers in the country, though undocumented migrants have started to replace Thai citizens in the Thai sex industry. Northeastern Thailand, sharing borders with Lao PDR and Cambodia, has the second-highest level of HIV infection.

Much of Thailand's success in reducing new HIV infections has been thanks to widespread adoption of safer behaviours and rising health-seeking behaviour. Effective STI treatment linked to condom promotion has resulted in a dramatic decrease in both HIV and other STI infections. One of the highest-profile contributions to this success has been made by the 100% condom use campaign targetting young people visiting commercial sex establishments.

New infections in pregnant women (and their children) are now gradually falling. According to a 1998 study by the Global Orphan Project, around 480,000 children in Thailand had HIV-positive mothers, and over 34,000 had been orphaned by AIDS. It is estimated the number of orphans will reach around 225,000 by 2005.

One of the greatest dangers for Thailand now is complacency. There are signs that HIV/AIDS in Thailand is increasingly perceived as "yesterday's news" by the public, government and donors. Government spending on HIV/AIDS drugs and condoms has been cut, though other prevention programs have not yet been affected. Without sustained support and commitment, the potential exists for a major resurgence of HIV/AIDS.