Condom use remains important for sexually transmitted disease (STD) prevention. This analysis examined the prevalence of problems with condoms among 1,152 participants who completed a supplemental questionnaire as part of Project RESPECT, a counseling intervention trial conducted at five publicly funded STD clinics between 1993 and 1997. Altogether, 336 participants (41%, 95% confidence interval: 38, 45) reporting condom use indicated that condoms broke, slipped off, leaked, or were not used throughout intercourse in the previous 3 months. Correspondingly, 8.9% (95% confidence interval: 7.0, 9.5) of uses resulted in STD exposure if partners were infected because of delayed application of condoms (4.3% of uses), breakage (2.0%), early removal (1.4%), slippage (1.3%), or leakage (0.4%). Use problems were significantly associated with reporting inconsistent condom use, multiple partners, and other condom problems. One-hundred thirty participants completing the questionnaire were tested for gonorrhea and chlamydia at this time and also 3 months earlier. Twenty-one (16.2%) were infected with incident gonorrhea and chlamydia, with no infections among consistent users reporting no use problems. Exact logistic regression revealed a significant dose-response relation between increased protection from condom use and reduced gonorrhea and chlamydia risk (p(trend) = 0.032). Both consistency of use and use problems must be considered in studies of highly infectious STD to avoid underestimating condom effectiveness.