PIP: HIV infection and AIDS in Thailand had been increasing among injecting drug users (IDU) from approximately 1% reported in 1987 to over 40% in late 1988. Two genetically different viruses have been shown to cause HIV infection among IDU and individuals infected sexually. HIV subtype B is responsible for 67-78% of infections in IDU patients, while HIV subtype E is responsible for 96% of infections in sexually acquired cases. It appears that 2 separate epidemics are occurring simultaneously. It has recently been noted that the HIV subtype E frequency in the IDU individuals seems to be increasing. A stable, but high, HIV prevalence rate of 40% among IDU in Bangkok was found. The incidence, however, is higher than observed in IDU individuals from Europe or North America. This is believed to be related to the high turnover rate (70%) of IDU individuals seeking the Thai government's detoxification treatment program. This study also compared 2 methods for estimating seroconversion. Method one assumed that the study subjects had an equal risk of HIV seroconverting on any day between blood testing (midpoint analysis) and produced an artificially low incidence estimate at both the beginning and end of a study interval. Method two assigned an equal probability for each day of the study interval for the patient to seroconvert. According to the author, this produced a more realistic estimate of the incidence curve. The IDU infections are important but are generally considered a minor part of the HIV infection epidemic which is occurring in Thailand. Thailand's HIV-positive population offers an opportunity for a field study of vaccines, as well as an opportunity to field test and evaluate prevention methods and strategies.