Results indicated that BAC estimation accuracy was negatively related to amount of alcohol consumed. That is, as BAC estimation accuracy decreased, alcohol consumption increased. Feedback from personal breathalyzer use increased BAC estimation accuracy over the first four events, but only at the lower BAC levels. Both heavy and light drinkers over-estimated their BAC levels, but light drinkers tended to over-estimate BAC levels to a greater degree than heavy drinkers. The findings are useful for education and treatment programs for heavy drinkers who routinely reach BAC levels above the legal driving limits but have a high probability of inaccurately estimating their true BAC levels. Participants were 19 individuals recruited from posters and college classroom announcements. Using an e-mail prompted Web-based questionnaire, participants reported estimations of their prior day BAC levels and subsequently measured their BAC levels over the course of 27 days. Fourteen participants reported consuming alcohol during the study period. Data were analyzed using descriptive and summary statistics. Tables, references