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To assess the impact of the HIV epidemic on the demographic development of the Thai population.A deterministic mathematical model was used to predict simultaneously epidemiological and demographic processes. Partial differential equations express the relationships between biological, behavioural and demographic variables. The model allows the evaluation of different sexual mixing patterns, variable transmission probabilities and incubation times. Validity analysis was performed by generating antecedent HIV prevalence patterns among military recruits and pregnant women.On the national level in Thailand we predict that the cumulative number of people in Thailand with HIV infection will exceed 1 million by 1999; the number of deaths from AIDS will be 550 000 by the year 2000 but will not reach 1 million until after the year 2014. Without the HIV epidemic the population growth rate was estimated at 1.3% per annum until 1995, after which a decline to 0.9% by 2005 is predicted. The HIV epidemic started to affect the population growth rate by 0.026% per year in 1991, and the difference is predicted to rise to about 0.12% per year during the period 1995–2000, to decline to 0.06% in 2005 and then to disappear. In the mid-1990s HIV affected mainly the 15–35-year-old age group, but over time younger and older age groups have been affected as a result of perinatal transmission, and a decline in fertility as well as ageing of the 15–35-year-old birth cohort. Because of HIV, in 2000 there will be 612 000 (1%) fewer people than expected and by 2010, 1 140 000 fewer (1.6%). We predict that the demographic impact of the HIV epidemic in the northern region will follow the same pattern, but with greater severity. Here, the effect on the population growth rate and the population age distribution is likely to be twice as high as at the national level.It is estimated that 1 million Thais will be infected with HIV by the year 2000 and an almost equal number will have died of AIDS by the year 2014. Although these numbers seem high, their direct and indirect effects on the demographic structure of the Thai population are small. However, at a regional level, for example in the northern region, the effect of the HIV epidemic may be more severe.