To identify socio-economic and demographic factors related to prevalent HIV-1 infection among female commercial sex workers (CSW) in Thailand oral interviews and blood samples were taken from 800 female commercial sex workers in northern and southern Thailand during a cross-sectional survey in 1992. The overall HIV-1 prevalence rate was 22% and showed a statistically significant decrease from 36% when the age at start of commercial sex work was between 12 and 15 years old to 11% when the age at start was 21 years or over. Working in direct service, working in the north, not being Thai, lower education, having no children and having a debt to the employer were all related to an elevated risk for HIV-1 infection in univariate analysis. In multivariate analysis younger age at start of commercial sex work, working in direct service, working in the north and having a debt to the employer were independently associated with prevalent HIV-1 infection. Prevention activities are urgently needed to prevent younger girls from entering sexual service business and to protect them from HIV 1 infection once they start working in the commercial sex service.