This study examines the effects of self-tested blood alcohol concentration (BAC) feedback, from personal hand-held breathalyzers, on the accuracy of BAC estimation. Using an e-mail prompted web-based questionnaire, 19 participants were asked to report both BAC estimates and subsequently measured BAC levels over the course of 27 days. Results from the 14 subjects who reported drinking during that time period suggest that BAC estimation improves over the first four drinking events, only when controlling for amount of alcohol consumed. BAC estimate accuracy was found to decrease as number of drinks and measured BAC increased. Participants were more likely to over-estimate their BAC's than to under-estimate them but this trend was much more pronounced for light drinkers than for heavy drinkers. There were no additional effects of heavy/light drinker status on estimate accuracy, beyond the effects of BAC at time of measured event. (Contains 6 tables.) [This study was funded by a grant from the UNCW Crossroads Program.]