A total of 72 social drinkers between the ages of 20 and 57 years participated in an ad lib social drinking session. At various intervals throughout the session participants provided estimates of their blood alcohol concentration (BAC) along with breath samples for objective determination of their BAC. Participants were classified into three groups, based on the pattern of their BAC estimation errors-Underestimators, Overestimators, or Mixed Pattern estimators. Underestimators consumed more alcohol and attained higher BACs during the drinking session than the other two groups. Underestimators also rated their level of intoxication significantly lower than other groups and were most likely to judge themselves fit to drive when their actual BAC was in excess of the statutory limit.