PIP: There were very few AIDS cases reported in Thailand as of 1988, where HIV was introduced relatively late in the course of the AIDS pandemic. Thailand was therefore classified as an epidemiologic pattern III country with regard to the HIV/AIDS pandemic. Also in 1988, however, Thailand experienced a major and rapid increase in HIV prevalence among IV drug users (IVDU). The Thai experience with HIV after the rapid spread first among IVDUs has been successive waves of HIV transmission to female prostitutes, then to their non-IVDU male clients, and then into the non-prostitute wives and girlfriends of these latter men in the general population. Three years after being declared a pattern III country, 300,000 people in Thailand were estimated to be infected out of a population of 55 million. Reasons for this unprecedented rapid spread of HIV infection may eventually come from research on sexual behavior and related diseases given the lack of evidence for human host genetic factors or particularly virulent etiologic agent factors to explain the phenomenon. The reason and dynamics behind the timing and rapidity of the 1988 epidemic among IVDUs for now remains unknown. The authors note that the scenario of HIV transmission observed in Thailand also seems to be unfolding in neighboring countries. HIV infection among female prostitutes and heterosexual men is consistently highest in the northern Thai provinces adjacent to Myanmar and Laos. This paper reviews the epidemiology and prevention of HIV infection and AIDS in Thailand, updating previous reports and commentary, and including previously unpublished or not widely available data.