PIP: Samples of peripheral blood mononuclear cells from 50 HIV-1-infected individuals in South Vietnam were analyzed to determine with which HIV-1 subtype the subjects were infected. Participants were from Ho Chi Minh city and five surrounding provinces; 16 samples from female prostitutes, 32 from IV drug users, and one each from a man and woman not in any HIV risk group. 32 individuals were therefore most likely infected by IV drug use and the rest through sexual contacts. PCR amplification and heteroduplex mobility assay found all but one case to be infected with HIV-1 subtype E. The only nonsubtype E infection was HIV-1 subtype B in a woman sexually infected by her seropositive partner who was most likely exposed to the virus in Europe. HIV-1 subtype E strongly predominates in South Vietnam. The homogeneous geographic distribution of subtype E suggests the recent introduction of the virus into the country. A Thai origin can be considered given the genetic relationship between the Thai and Vietnamese subtypes E. It may be assumed that subtype E infections of Vietnamese prostitutes are related to the progressive entry and spread of HIV-1 subtype E from Thailand to Cambodia and then to southern Vietnam.