OBJECTIVE Given that most effective alcohol harm-reduction laws specify the blood alcohol concentration (BAC) that constitutes illegal behavior (e.g., the .08% breath alcohol concentration legal limit), interventions that allow drinkers to accurately estimate their BACs, and thus better assess their risk, have potential importance to long-term driving-under-the-influence prevention efforts. This study describes a field experiment designed to test the impact on drinking of providing "Know Your Limit" (KYL) BAC estimation cards to individuals in a natural drinking environment. METHOD We randomly sampled 1,215 U.S. residents as they entered Mexico for a night of drinking, interviewed them, and randomly assigned them to one of six experimental conditions. Participants were reinterviewed and breath-tested when they returned to the United States. The experimental conditions included providing generic warnings about drinking and driving, giving out gender-specific BAC calculator cards (KYL cards), and providing incentives to moderate their drinking. RESULTS Cueing participants about the risks of drunk driving resulted in significantly lower BACs (relative to control) for participants who indicated that they would drive home. Providing KYL matrixes did not reduce BACs, and, in fact, some evidence suggests that KYL cards undermined the effect of the warning. CONCLUSIONS KYL information does not appear to be an effective tool for reducing drinking and driving. Implications for prevention and future research are discussed.