There is compelling evidence that male condoms effectively prevent transmission of sexual pathogens, including HIV-1. Condom breakage and slippage reduce this effect. We measured rates of condom slippage and breakage during heterosexual commercial sex in northern Thailand. Of 7,594 condoms examined in 4,734 client visits (5,040 sex acts), breakage was noted in 1.8% of single condom use (49.3% of acts), as compared with 0.2% with two condoms(49% of sex acts). and no breaks with more than two condoms (1.2% of sex acts). These breakage rates declined from 5.9% in a similar 1992 study in which 2.8% of sex acts were with more than one condom used at a time. Slippage occurred in only 0.1% of sex acts. Case-control analysis indicated that multiple clients, younger aged clients, sex after midnight, and high intensity (rough) sex were associated with condom breaks. The decline in breakage may be attributable to greater expertise in condom use by sex workers and clients, in response to the successful Thai national "100% Condom Campaign." Use of more than one condom during sex has been initiated by sex workers and their clients, a community response to condom promotion messages and fears of HIV infection. These data demonstrate the potential of condom use for high efficacy in reducing exposure to HIV-1 and other STDs. If condom use in commercial sex remains high, HIV incidence may decline among Thai men.