To assess the extent to which condoms are used effectively in commercial heterosexual intercourse. Data on the number of condoms that had broken or slipped off, the sexual technique during which this had occurred and the perceived cause of failure were collected. The use of non-water-soluble lubricants and non-fortified condoms during anal intercourse, and the demand for a greater variety of condom sizes were also examined. One hundred and twenty-seven female prostitutes and 91 male clients from different parts of The Netherlands were interviewed face-to-face between July 1990 and March 1991. Of those who used condoms during vaginal intercourse, 49% of the prostitutes had experienced condom breakage in the previous 6 months, and 16% of the clients in the previous 12 months. The breakage rate was 0.8% for prostitutes and 1.5% for clients. Condom quality was seldom reported as the cause; breakage was generally attributed to human factors, such as rough or prolonged intercourse, incorrect handling of the condom or the use of insufficient lubricant. Prostitutes also identified penis size as a cause. Condoms slipping off before or after ejaculation was reported less frequently than breakage. Thirteen per cent of clients and 36% of prostitutes expressed a need for either smaller or larger condoms. Of the prostitutes, 9% used oil or vaseline as a lubricant. In view of the low rate of condom failure in heterosexual prostitution in The Netherlands, the potential spread of HIV by this means is small. The use of a greater variety of condom sizes may further reduce the failure rate. Few prostitutes remain ignorant about the adverse effects of oil-based lubricants on condoms.