The Thai government began an HIV-control programme in 1989. The programme had the following parts: the government bought and distributed sufficient condoms to protect much of the commercial sex in the country; sanctions were brought against commercial sex establishments where condoms were not used consistently; and a media campaign bluntly advised men to use condoms with prostitutes. Between 1989 and 1993 the use of condoms in commercial sex in Thailand increased from 14 to 94%, according to surveys of prostitutes, and the number of cases of the five major sexually transmitted diseases declined by 79% in men. We estimate that sex acts with prostitutes where there was a risk of HIV transmission declined from about 2.6% in June, 1989, to about 1.6% in June, 1993. If condom use in commercial sex stays high, future cohorts of young men and women may experience lower HIV incidence rates than those of the recent past. However, although condom use is high, there are many more infected prostitutes than before and many infected men who will pass HIV to their wives.