Background and Objectives: Mechanical failure may reduce the efficacy of condoms. Little is known about frequency and determinants of condom failure in groups at high risk of sexually transmitted diseases (STD). Goal: To measure condom breakage and slippage rates and evaluate potential determinants of failure among women attending a public STD clinic. Study Design: Women attending an STD clinic participated in a 6-month prospective study of barrier contraception for the prevention of STD. They completed sexual diaries that were reviewed at monthly follow-up visits. No data were collected from the male partners. Baseline characteristics of the participants and time-dependent behaviors were evaluated as potential determinants of condom failure. Results: Of 21,852 condoms used by 892 women, 500 broke during intercourse (2.3%) and 290 slipped (1.3%). Breakage was more common among young, black, single nulliparae who engaged in high-risk behavior. Slippage was more common among married women with children. Failure rates decreased with condom use, with coital frequency, and with use of spermicides. Conclusion: User characteristics and experience are determinants of breakage and slippage, which are often regarded only as the effect of product design flaws. Attention to modifiable determinants of failure may improve user counseling and product labeling.